Free ASCP MLS Exam Practice Questions Mock Test: Part 24 – Microbiology – Gram-Negative Bacilli Identification

Klebsiella pneumoniae is distinguished from Escherichia coli primarily by being indole-negative, whereas E. coli is indole-positive.

• a) Lactose fermenter – Both K. pneumoniae and E. coli ferment lactose (though K. pneumoniae may do so more slowly).

• c) Oxidase-negative – Both are oxidase-negative (not positive).

• d) Non-motile – K. pneumoniae is non-motile, while E. coli is usually motile.

1. Swarming on blood agar – Proteus mirabilis exhibits characteristic concentric waves of swarming motility due to its highly active flagella.

2. Urease-positive – Proteus species rapidly hydrolyze urea, producing ammonia (alkaline pH → pink color in urea agar).

Why Not the Others?

• a) Salmonella enterica

  • Does not swarm on blood agar.

  • Usually urease-negative (except some rare strains).

• c) Shigella flexneri

  • Non-motile (no swarming).

  • Urease-negative.

• d) Acinetobacter baumannii

  • Non-motile (no flagella → no swarming).

  • Urease-negative (or weakly positive in some strains).

1. Oxidizes lactose:

   • Unlike most non-fermenters, it slowly oxidizes lactose in O-F medium.

2. Yellow pigment:

   • Produces a bright yellow pigment (flexirubin-type) on solid media.

3. Clinical relevance:

   • Causes meningitis in neonates and sepsis in immunocompromised adults.

   • Intrinsically resistant to many antibiotics (e.g., carbapenems, aminoglycosides).

Why Not the Others?

• (a) Pseudomonas aeruginosa:

  • Oxidizes glucose (not lactose) and produces pyocyanin (blue-green pigment).

• (b) Serratia marcescens:

  • Ferments lactose weakly and produces red pigment (prodigiosin) at 25°C.

• (d) Stenotrophomonas maltophilia:

  • Does not oxidize lactose and is non-pigmented.

1. Phenylalanine deaminase (PAD) positive:

   • Hydrolyzes phenylalanine to phenylpyruvic acid, turning green when reacted with ferric chloride (FeCl₃).

   • Classic for Proteus, Morganella, and Providencia (the “PAD-positive” group).

2. Additional traits of Proteus vulgaris:

   • Urease-positive, H₂S-positive (blackening in TSI), and motile (swarming on blood agar).

Why Not the Others?

• (a) Klebsiella oxytoca: PAD-negative (but indole-positive, unlike K. pneumoniae).

• (c) Serratia marcescens: PAD-negative (known for red pigment at 25°C).

• (d) Citrobacter freundii: PAD-negative (but H₂S-positive, resembling Salmonella).

• In cystic fibrosis (CF) patients, chronic respiratory infections are very common due to thick mucus and impaired clearance.

• The most common pathogen is Pseudomonas aeruginosa, but another important and emerging pathogen in CF is Stenotrophomonas maltophilia.

• It is a non-fermenting Gram-negative bacillus, often multidrug-resistant, and can complicate management in CF patients.

• Other options:

  • Proteus mirabilis → associated with UTIs, not CF lungs.

  • Serratia marcescens → opportunistic, produces red pigment, but less associated with CF.

  • Salmonella enterica → causes gastroenteritis/typhoid, not respiratory disease.

• Acinetobacter baumannii is a non-motile, glucose-oxidizing (oxidative metabolism), Gram-negative bacillus.

• It is a notorious cause of nosocomial (hospital-acquired) pneumonia, especially in ventilated patients.

• Key traits:

  • Non-lactose fermenter (pale colonies on MacConkey agar).

  • Oxidase-negative (differentiates from Pseudomonas).

  • Often multidrug-resistant (e.g., carbapenem-resistant strains).

Why Not the Others?

• (a) Escherichia coli:

  • Ferments glucose (not oxidative), motile, and typically causes UTIs/enteric infections.

• (c) Klebsiella pneumoniae:

  • Ferments lactose (pink MacConkey colonies), non-motile, but metabolizes sugars fermentatively.

• (d) Pseudomonas aeruginosa:

  • Oxidative (like Acinetobacter), but motile and oxidase-positive.

Klebsiella pneumoniae is a Gram-negative bacillus notorious for causing pneumonia in alcoholics and other immunocompromised individuals (e.g., diabetics, elderly).

Key Features of K. pneumoniae Pneumonia:

1. “Currant jelly” sputum (thick, bloody, and mucoid due to capsular polysaccharide).

2. Lung abscesses and cavitation (destructive lung lesions).

3. Alcoholics at high risk: Impaired immune function and aspiration predisposition.

Why Not the Others?

• a) Pseudomonas aeruginosa: More common in cystic fibrosis or hospital-acquired pneumonia (not specifically linked to alcoholism).

• c) Escherichia coli: Rarely causes primary pneumonia; more associated with UTIs.

• d) Proteus mirabilis: Primarily linked to UTIs (urease-positive) and wound infections.

• Burkholderia pseudomallei → causes melioidosis, an endemic disease in Southeast Asia & Northern Australia.

• Acquired from soil and water exposure, especially in diabetics and farmers.

• Can present as:

  • Pneumonia

  • Abscesses (multiple organs)

  • Sepsis

• Other options:

  • Pseudomonas aeruginosa → CF pneumonia, wound/UTI, but not melioidosis.

  • Acinetobacter baumannii → nosocomial infections, ventilator-associated pneumonia.

  • Stenotrophomonas maltophilia → opportunistic nosocomial pathogen, multidrug resistant.

Salmonella enterica produces black colonies on Hektoen enteric (HE) agar due to hydrogen sulfide (H₂S) production, which reacts with ferric ammonium citrate in the medium.

Key Traits of Salmonella on HE Agar:

1. Black centers (H₂S-positive).

2. Blue-green colonies with black precipitates (lactose non-fermenter).

3. Differentiates from Shigella (H₂S-negative, green colonies).

Why Not the Others?

• a) Escherichia coli: Pink/orange colonies (lactose fermenter, H₂S-negative).

• c) Shigella flexneri: Green colonies (lactose non-fermenter, H₂S-negative).

• d) Klebsiella oxytoca: Yellow/orange colonies (lactose fermenter, H₂S-negative).

• Cefsulodin-Irgasan-Novobiocin (CIN) agar is a selective medium designed to isolate Yersinia enterocolitica from stool samples.

• It suppresses normal gut flora while allowing Yersinia to form bull’s-eye colonies (dark red centers with transparent borders).

• Other options:

  • (b) Vibrio parahaemolyticus – Requires TCBS agar.

  • (c) Campylobacter fetus – Grows on Campy-BAP agar (microaerophilic).

  • (d) Salmonella typhi – Cultured on XLD/SS agar.

Pseudomonas aeruginosa is well known for producing the blue-green pigment pyocyanin, which gives colonies and infected wound dressings a distinctive color. Additionally, some strains may produce:

• Pyoverdine (yellow-green fluorescent pigment under UV light).

• Pyorubin (red pigment, less common).

Why not the others?

• a) Red – Rare (only with pyorubin).

• c) Yellow – Seen in Staphylococcus aureus (carotenoids) or Serratia marcescens (prodigiosin at lower temps).

• d) Black – Seen in Burkholderia cepacia or Aspergillus niger.

Yersinia enterocolitica produces distinctive “bull’s-eye” colonies on Cefsulodin-Irgasan-Novobiocin (CIN) agar, a selective medium for isolating Yersinia from stool samples.

Key Traits of Y. enterocolitica:

1. Gram-negative coccobacillus (cold-tolerant, grows at 4°C).

2. Causes yersiniosis:

   • Symptoms: Diarrhea, abdominal pain (mimics appendicitis).

   • Transmission: Contaminated pork, unpasteurized milk.

3. Virulence factors:

   • Invasin (invades intestinal cells).

   • YOPs (Yersinia outer proteins, suppress immune response).

Why Not the Others?

• b) Salmonella Typhi: Forms colorless colonies on CIN (not bull’s-eye).

• c) Shigella flexneri: Inhibited by CIN agar.

• d) Vibrio parahaemolyticus: Grows on TCBS (thiosulfate-citrate-bile salts-sucrose agar), not CIN.

Escherichia coli strains possessing the K1 capsular antigen are a leading cause of neonatal meningitis, rivaling Group B Streptococcus (GBS) in incidence.

Key Traits of E. coli K1:

1. K1 polysaccharide:

   • Mimics human neural cell adhesion molecules (evades immune detection).

   • Enhances blood-brain barrier penetration.

2. Transmission: Vertical (maternal vaginal colonization during birth).

3. Clinical impact:

   • High mortality (~15%) and neurologic sequelae in survivors.

Why Not the Others?

• b) Klebsiella pneumoniae: Causes pneumonia/UTIs (rarely neonatal meningitis).

• c) Serratia marcescens: Nosocomial infections (not linked to K1 antigen).

• d) Proteus mirabilis: UTIs/stones (urease-positive; no K1 antigen).

• Serratia marcescens is known for producing a red pigment (prodigiosin) when cultured at room temperature (25°C).

• This pigment is less pronounced or absent at 37°C, making it a key diagnostic feature.

• Other options:

  • (a) Pseudomonas aeruginosa – Produces pyocyanin (blue-green pigment).

  • (c) Enterobacter aerogenes – Non-pigmented.

  • (d) Proteus mirabilis – Non-pigmented, swarms on blood agar.

The question describes an organism that requires X factor (hemin) and V factor (NAD or NADP) for growth.

Breakdown of options:

• Haemophilus influenzae (Correct answer)

  • Requires both X and V factors for growth.

  • Grows on chocolate agar (supplies X and V factors) but not on blood agar unless supplemented (e.g., with Staphylococcus streak for V factor in satelliting test).

• Neisseria meningitidis

  • Does not require X or V factors.

  • Grows on Thayer-Martin agar (modified chocolate agar with antibiotics).

• Bordetella pertussis

  • Requires special media (Bordet-Gengou or Regan-Lowe agar) but not X or V factors.

• Legionella pneumophila

  • Requires cysteine (supplied in buffered charcoal yeast extract [BCYE] agar) but not X or V factors.

Aeromonas hydrophila is a Gram-negative bacillus that causes “red leg” syndrome, a severe skin and soft tissue infection in fish handlers or individuals exposed to contaminated water.

Key Traits of A. hydrophila:

1. Waterborne pathogen: Found in freshwater (lakes, rivers) and marine environments.

2. Virulence factors:

   • Hemolysins (cause tissue necrosis).

   • Exotoxins (contribute to wound infections).

3. Clinical manifestations:

   • “Red leg”: Cellulitis with hemorrhagic bullae.

   • Septicemia (in immunocompromised patients).

Why Not the Others?

• a) Vibrio parahaemolyticus: Causes seafood-associated gastroenteritis (not red leg).

• c) Pseudomonas aeruginosa: Linked to “hot tub folliculitis” or burn infections (not fish handlers).

• d) Proteus vulgaris: Associated with UTIs and wound infections (not aquatic exposure).

Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar is a selective and differential medium specifically designed for isolating Vibrio species, including:

• Vibrio cholerae (causative agent of cholera)

• Vibrio parahaemolyticus

• Vibrio vulnificus

Why Not the Others?

• a) Salmonella typhi → Grows on MacConkey, XLD, or SS agar, not TCBS.

• c) Campylobacter jejuni → Requires Campy-BAP or Skirrow’s agar (microaerophilic conditions).

• d) Helicobacter pylori → Grows on blood or chocolate agar in microaerophilic conditions.

• Satellite phenomenon refers to the growth of Haemophilus influenzae around colonies of Staphylococcus aureus (or other bacteria) on blood agar.

• S. aureus releases V factor (NAD/NADP) during growth, which H. influenzae requires but cannot synthesize on its own.

• This results in small H. influenzae colonies appearing only near the S. aureus streaks (“satelliting”).

Why Not the Others?

• (a) Streptococcus pneumoniae – Grows independently; no satellite phenomenon.

• (c) Neisseria gonorrhoeae – Requires Thayer-Martin agar, not V factor.

• (d) Staphylococcus aureus – It provides V factor but does not exhibit satelliting itself.

Oxidase-positive bacteria produce cytochrome c oxidase, a key enzyme in their electron transport chain. Among the options:

• c) Pseudomonas aeruginosa – Oxidase-positive (a defining feature of this organism).

• a) Escherichia coli – Oxidase-negative.

• b) Klebsiella pneumoniae – Oxidase-negative.

• d) Proteus mirabilis – Oxidase-negative.

1. Oxidase-positive – Rules out Stenotrophomonas maltophilia (oxidase-negative).

2. Ferments glucose in OF (Oxidative-Fermentative) medium – Confirms fermentative metabolism (unlike Pseudomonas or Alcaligenes, which oxidize glucose).

3. Additional traits of Aeromonas:

   • Catalase-positive, motile, and resistant to vibriostatic agent O/129.

   • Causes wound infections, septicemia, and diarrhea (waterborne exposure).

Why Not the Others?

• (a) Alcaligenes faecalis: Oxidase-positive but does not ferment glucose (strictly oxidative).

• (b) Pseudomonas fluorescens: Oxidase-positive but oxidizes glucose (non-fermenter).

• (d) Stenotrophomonas maltophilia: Oxidase-negative and glucose metabolism is weak/non-fermentative.

• Microaerophilic (5% O₂, 10% CO₂) – Campylobacter jejuni requires reduced oxygen and elevated CO₂.

• 42°C – Optimal growth temperature (selective for C. jejuni in stool cultures).

• Other options are incorrect:

  • (a) Aerobic – C. jejuni does not grow in normal air.

  • (c) Anaerobic – Incorrect oxygen requirement.

  • (d) Capnophilic – Needs microaerophilic, not just high CO₂.

• Escherichia coli is:

  • Indole-positive → produces indole from tryptophan

  • Oxidase-negative → typical of Enterobacteriaceae

  • Lactose fermenter → produces pink colonies on MacConkey agar

• Other options:

  • Pseudomonas aeruginosa → oxidase-positive, non-lactose fermenter

  • Klebsiella pneumoniae → lactose fermenter, oxidase-negative, indole-negative (except K1 strains)

  • Proteus mirabilis → oxidase-negative, indole-negative, non-lactose fermenter

Acinetobacter baumannii is notorious for resistance to colistin (a last-resort antibiotic) and is a frequent cause of hospital outbreaks, particularly in ICUs and burn units.

Key Traits of A. baumannii:

1. Extreme Drug Resistance:

   • Resistant to carbapenems, colistin, and multiple other antibiotics.

   • Often labeled “CRAB” (Carbapenem-Resistant Acinetobacter baumannii).

2. Survivability: Persists on surfaces (e.g., ventilators, catheters) for weeks.

3. Infections: Pneumonia, bloodstream infections, wound infections.

Why Not the Others?

• a) Klebsiella pneumoniae: Can be colistin-resistant but less environmentally hardy.

• b) Pseudomonas aeruginosa: Often MDR but less prone to outbreaks.

• d) Serratia marcescens: Less commonly resistant to colistin.

1. Bipolar staining (“safety pins”):

   • Under Wright-Giemsa or Wayson stain, the bacilli show dark blue/purple poles with a lighter central area, resembling old-fashioned safety pins.

2. Gram-negative, non-motile, and facultative anaerobic.

3. Causes plague:

   • Bubonic plague (swollen lymph nodes/buboes).

   • Pneumonic plague (person-to-person transmission via aerosols).

Why Not the Others?

• (b) “Curved rods”: Describes Vibrio or Campylobacter.

• (c) “Chains”: Typical of Streptococcus or Bacillus.

• (d) “Clusters”: Seen in Staphylococcus.

1. Grows on MacConkey agar as a non-lactose fermenter (pale/colorless colonies).

2. Lysine decarboxylase-positive – Critical biochemical test to differentiate Salmonella from other Enterobacteriaceae.

3. Additional traits:

   • H₂S-positive (black centers on TSI or HE agar).

   • Motile (unlike Shigella).

Why Not the Others?

• (b) Proteus mirabilis:

  • Lysine decarboxylase-negative but ornithine decarboxylase-positive.

  • H₂S-positive and swarms on blood agar.

• (c) Morganella morganii:

  • Lysine decarboxylase-negative but ornithine decarboxylase-positive.

  • Phenylalanine deaminase (PAD)-positive (green with FeCl₃).

• (d) Providencia stuartii:

  • Lysine decarboxylase-negative but ornithine decarboxylase-variable.

  • PAD-positive and associated with UTIs in catheterized patients.

Vibrio vulnificus is a halophilic (salt-loving), Gram-negative bacterium notorious for causing severe wound infections and septicemia after exposure to seawater or consumption of raw shellfish (e.g., oysters).

Why Not the Others?

• b) Salmonella Typhi – Causes typhoid fever (food/waterborne, not seawater-linked).

• c) Shigella sonnei – Causes dysentery (fecal-oral transmission).

• d) Escherichia coli – Associated with UTIs/gastroenteritis (not seawater wounds).

Acinetobacter baumannii is a Gram-negative coccobacillus with the following key traits:

1. Oxidase-negative (distinguishes it from Pseudomonas and other non-fermenters).

2. Non-motile (lacks flagella).

3. Non-lactose fermenter (forms pale colonies on MacConkey agar).

Why Not the Others?

• a) Oxidase-positive: Incorrect (Acinetobacter is oxidase-negative, unlike Pseudomonas).

• b) Motile: Incorrect (Acinetobacter is non-motile).

• d) Lactose fermenter: Incorrect (it does not ferment lactose).

• The string test (also called “vibrio string reaction”) is a rapid screening test for Vibrio cholerae.

• When mucoid colonies are mixed with sodium deoxycholate, V. cholerae lyses and releases DNA, forming a viscous “string” when pulled with a loop.

• Not used for the other options:

  • (b) Helicobacter pylori – Diagnosed by urease test/biopsy.

  • (c) Bacillus anthracis – Identified by Gram stain, capsule, and motility tests.

  • (d) Yersinia pestis – Relies on bipolar staining and culture.

• Salmonella typhi (causative agent of typhoid fever) is classically associated with:

  • Step-ladder fever pattern

  • Rose spots (faint, salmon-colored macules on the abdomen and chest)

  • Hepatosplenomegaly

  • Intestinal perforation (severe cases)

• “Rose spots” occur due to bacterial emboli in skin capillaries.

• Other options:

  • Shigella dysenteriae → causes bacillary dysentery (bloody diarrhea), no rose spots.

  • Vibrio vulnificus → wound infections/septicemia, not typhoid features.

  • Escherichia coli → causes diarrhea, UTIs, meningitis, but not rose spots.

Pseudomonas aeruginosa is known for producing a characteristic fruity or grape-like odor in culture due to its production of 2-aminoacetophenone, a volatile compound.

Key Traits of P. aeruginosa:

1. Gram-negative, aerobic bacillus with polar flagella (motile).

2. Pigment production:

   • Pyocyanin (blue-green).

   • Pyoverdine (yellow-green, fluorescent under UV light).

3. Opportunistic pathogen: Causes infections in burns, cystic fibrosis (pneumonia), and UTIs.

Why Not the Others?

• b) Salmonella enterica: No distinctive odor (associated with gastroenteritis).

• c) Shigella sonnei: No distinctive odor (causes dysentery).

• d) Acinetobacter baumannii: Often odorless (nosocomial infections).

1. Indole-positive:

   • Produces indole from tryptophan (tested with Kovac’s reagent).

2. Motile:

   • Has peritrichous flagella (unlike Shigella or Klebsiella).

3. Additional traits:

   • Lactose fermenter (pink colonies on MacConkey agar).

   • Oxidase-negative.

Why Not the Others?

• (a) Klebsiella pneumoniae:

  • Indole-negative (except K. oxytoca).

  • Non-motile.

• (b) Shigella flexneri:

  • Indole-positive (variable), but non-motile.

• (d) Salmonella typhi:

  • Indole-negative and motile.

1. Fastidious growth requirements:

   • Requires cysteine and iron for growth (provided in cysteine-enriched media like BCYE agar or Thayer-Martin agar with cysteine).

   • Does not grow on standard blood or MacConkey agar.

2. Zoonotic pathogen:

   • Causes tularemia (“rabbit fever”), transmitted via ticks, rabbits, or aerosols.

3. Safety note:

   • Highly infectious (BSL-3 organism); lab culture is rarely attempted.

Why Not the Others?

• (a) Blood agar: Lacks cysteine; F. tularensis does not grow.

• (b) Chocolate agar: May support growth if supplemented with cysteine, but not standard.

• (d) MacConkey agar: Inhibits Francisella (no growth).

• Oxidase-positive (key feature of Pseudomonas).

• Produces pyocyanin (blue-green pigment, unique to P. aeruginosa).

• Other options:

  • Stenotrophomonas (oxidase-negative).

  • Acinetobacter (oxidase-negative).

  • Serratia (oxidase-negative, produces red pigment prodigiosin).

• Legionella pneumophila is a fastidious organism that requires:

  • L-cysteine and iron (provided in BCYE agar).

  • Activated charcoal (neutralizes toxic metabolites).

• BCYE agar is selective for Legionella and supports its growth when supplemented with antibiotics (e.g., GVPC for contamination prevention).

• Other media are unsuitable:

  • (a) MacConkey agar – Lacks cysteine/iron; Legionella does not grow.

  • (c) Hektoen enteric agar – For Salmonella/Shigella (inhibits Legionella).

  • (d) Chocolate agar – Supports Haemophilus/Neisseria but lacks charcoal/cysteine.

Bordetella pertussis, the causative agent of whooping cough (pertussis), is a fastidious bacterium that requires specialized media for isolation. Regan-Lowe agar (containing charcoal, horse blood, and antibiotics) is one of the primary selective media used for its cultivation.

Why Not the Others?

• a) Blood agar – Not selective; B. pertussis may grow poorly or be overgrown by other flora.

• c) MacConkey agar – Inhibits Bordetella (does not support its growth).

• d) TCBS agar – Used for Vibrio spp., not Bordetella.

The most reliable distinguishing feature between Shigella and Salmonella is motility:

• Salmonella: Motile (peritrichous flagella).

• Shigella: Non-motile (lacks flagella).

Supporting Tests for Differentiation:

1. Lactose fermentation (a):

   • Both are typically non-lactose fermenters (but S. sonnei ferments lactose slowly).

2. Indole test (c):

   • Shigella dysenteriae is indole-positive, but other species vary.

   • Salmonella is indole-negative.

3. Oxidase reaction (d):

   • Both are oxidase-negative.

Vibrio cholerae grows optimally on Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar, a selective and differential medium that:

• Selects for Vibrio spp. (high pH inhibits most other bacteria).

• Differentiates V. cholerae via yellow colonies (sucrose fermentation).

Why Not the Others?

• a) MacConkey agar – Supports growth but does not differentiate Vibrio well (non-lactose fermenter, pale colonies).

• c) Blood agar – Supports growth but lacks selectivity (non-specific).

• d) Hektoen enteric agar – Used for Salmonella/Shigella, not Vibrio.

Haemophilus influenzae is a fastidious bacterium that requires both X and V factors for growth, which correspond to:

• X factor = Hemin (heat-stable, derived from hemoglobin).

• V factor = NAD (nicotinamide adenine dinucleotide) (heat-labile).

These factors are provided in culture media like chocolate agar (heated blood releases X and V factors) or supplemented blood agar.

Key Points:

1. X Factor (Hemin): Essential for cytochrome enzymes in aerobic respiration.

2. V Factor (NAD): Acts as a coenzyme in metabolic reactions.

3. Satellitism Test: H. influenzae grows near Staphylococcus streaks (which secrete NAD).

Why Not the Others?

• a) X and V factors – Correct but incomplete (they are hemin and NAD).

• b) Hemin and NAD – Correct but redundant (same as X and V).

• d) None – Incorrect (H. influenzae is fastidious)

While both Neisseria and Moraxella are oxidase-positive, Gram-negative cocci, they are differentiated by their ability to ferment carbohydrates:

• Neisseria gonorrhoeae ferments glucose (but not maltose/sucrose/lactose).

• Neisseria meningitidis ferments glucose and maltose.

• Moraxella catarrhalis does not ferment any carbohydrates.

Why Not the Others?

• a) Oxidase: Both are oxidase-positive (does not differentiate).

• c) Catalase: Both are catalase-positive (does not differentiate).

• d) Indole: Neither produces indole (irrelevant).

1. Intrinsic resistance to carbapenems (e.g., imipenem, meropenem):

   • Due to L1 metallo-β-lactamase and L2 serine-β-lactamase.

2. Other resistances:

   • Aminoglycosides (via modifying enzymes).

   • Most β-lactams (except trimethoprim-sulfamethoxazole, the drug of choice).

3. Clinical relevance:

   • Opportunistic pathogen causing pneumonia, bloodstream infections, and catheter-associated UTIs in hospitalized patients.

Why Not the Others?

• (a) Penicillin: Resistance is common but not unique (many Gram-negatives are resistant).

• (c) Aminoglycosides: Resistance is acquired (not intrinsic).

• (d) Vancomycin: Targets Gram-positive cell walls (irrelevant for Gram-negatives).

Pseudomonas aeruginosa is a non-fermenting Gram-negative bacillus, meaning it metabolizes carbohydrates oxidatively (via cytochrome c oxidase) rather than through fermentation.

Why Not the Others?

• a) Escherichia coli – Ferments lactose and glucose (Enterobacteriaceae).

• c) Klebsiella pneumoniae – Ferments lactose (pink on MacConkey agar).

• d) Proteus mirabilis – Ferments glucose but not lactose (swarms on blood agar).

1. Melioidosis:

   • A tropical disease (endemic in Southeast Asia, Northern Australia) causing pneumonia, abscesses, or sepsis.

   • Transmission via soil/water contact (inhalation, ingestion, or skin inoculation).

2. Lab ID:

   • Gram-negative bacillus, oxidase-positive, and bipolar staining (“safety pin” appearance).

   • Grows on Ashdown’s selective agar (creamy wrinkled colonies).

3. Treatment:

   • Ceftazidime or meropenem (intrinsically resistant to many antibiotics).

Why Not the Others?

• (b) Pseudomonas putida: Environmental, rarely pathogenic.

• (c) Aeromonas caviae: Causes diarrhea/wound infections (water exposure).

• (d) Chromobacterium violaceum: Rare; produces purple pigment and causes sepsis.

The urease test is key in differentiating Proteus from Salmonella:

• Proteus spp. (e.g., P. mirabilis) are urease-positive—they rapidly hydrolyze urea, producing ammonia and causing a pH shift (color change to pink in urea agar).

• Salmonella spp. are urease-negative (no color change).

Why not the others?

• b) Citrate: Both Proteus (usually negative, except P. mirabilis) and Salmonella (usually positive) can overlap.

• c) Oxidase: Both are oxidase-negative.

• d) Phenylalanine deaminase (PDA): Proteus is PDA-positive, but this test is less commonly used than urease for initial differentiation.

Proteus mirabilis is urease-positive, meaning it produces the enzyme urease, which hydrolyzes urea into ammonia and CO₂, raising the pH (turns agar pink in urea medium).

Why Not the Others?

• a) Escherichia coli – Urease-negative (does not split urea).

• c) Shigella flexneri – Urease-negative.

• d) Salmonella typhi – Urease-negative.

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of traveler’s diarrhea, particularly in visitors to tropical or developing regions.

Key Traits of ETEC:

1. Virulence factors:

   • Heat-labile (LT) and heat-stable (ST) enterotoxins stimulate fluid secretion in the intestines.

2. Symptoms:

   • Watery, non-bloody diarrhea.

   • Abdominal cramps, nausea.

3. Transmission:

   • Contaminated food/water (e.g., street food, untreated water).

Why Not the Others?

• a) Salmonella enterica: Causes enterocolitis (often from poultry/eggs).

• c) Shigella dysenteriae: Causes bloody diarrhea (dysentery).

• d) Vibrio vulnificus: Associated with seafood/seawater exposure.

Campylobacter jejuni requires microaerophilic conditions (3–5% O₂, 5–10% CO₂) for optimal growth, typically achieved using specialized gas packs or incubators.

Why Not the Others?

• a) Aerobic – C. jejuni cannot grow in normal atmospheric oxygen (21% O₂).

• c) Anaerobic – Requires some oxygen (microaerophilic, not anaerobic).

• d) Capnophilic – Needs low O₂ (not just high CO₂ like Neisseria).

• Serratia marcescens produces a characteristic red pigment called prodigiosin.

• This feature helps distinguish it from other Enterobacteriaceae.

• Other pigments (for comparison):

  • Pseudomonas aeruginosa → blue-green (pyocyanin)

  • Staphylococcus aureus → golden-yellow colonies

  • Chromobacterium violaceum → violet pigment

Klebsiella pneumoniae is renowned for producing mucoid colonies due to its thick polysaccharide capsule, which contributes to its virulence by evading phagocytosis.

Key Traits of K. pneumoniae:

1. Capsule:

   • Composed of complex polysaccharides (e.g., K1, K2 antigens).

   • Appears sticky and viscous on agar plates.

2. Clinical Impact:

   • Pneumonia (especially in alcoholics; “currant jelly” sputum).

   • UTIs and nosocomial infections.

Why Not the Others?

• b) Escherichia coli: Rarely mucoid (except in hypervirulent/encapsulated strains).

• c) Salmonella paratyphi: Non-mucoid (smooth, non-lactose-fermenting colonies).

• d) Shigella flexneri: Non-mucoid (dry, flat colonies on MacConkey agar)

• Helicobacter pylori is the primary cause of gastric ulcers and chronic gastritis.

• It is strongly urease-positive, which helps it survive stomach acid by producing ammonia (detected via rapid urease test, e.g., CLOtest).

• Other options are incorrect:

  • (a) Salmonella typhi – Causes typhoid fever (urease-negative).

  • (c) Shigella dysenteriae – Causes dysentery (urease-negative).

  • (d) Proteus vulgaris – Urease-positive but linked to UTIs, not gastric ulcers.

• Opportunistic pathogen primarily causing chronic respiratory infections in:

  • Cystic fibrosis (CF) patients (leads to declining lung function).

  • Immunocompromised individuals (e.g., pneumonia in ICU patients).

• Intrinsically antibiotic-resistant (survives in disinfectants and moist environments).

Why Not the Others?

• (a) Urinary tract infections → More typical of E. coli or Proteus.

• (c) Foodborne gastroenteritis → Associated with Salmonella, Campylobacter, etc.

• (d) Skin abscesses → Caused by Staphylococcus aureus or Pseudomonas aeruginosa.

Escherichia coli is the most common cause of urinary tract infections (UTIs) and is indole-positive, a key biochemical characteristic (detected via Kovac’s reagent in the indole test).

Why not the others?

• a) Proteus mirabilis – Associated with UTIs (especially with stones due to urease production) but is indole-negative.

• b) Klebsiella pneumoniae – Can cause UTIs but is indole-negative.

• d) Salmonella enterica – Primarily a gastrointestinal pathogen and indole-negative.

• Salmonella enterica:

  • Non-lactose fermenter → red slant (alkaline)

  • Glucose fermenter → yellow butt (acid)

  • H₂S producer → black precipitate in the butt

  • Gas may or may not be produced

• Other options:

  • Escherichia coli → lactose fermenter, indole-positive, no H₂S

  • Klebsiella pneumoniae → lactose fermenter, no H₂S

  • Enterobacter cloacae → lactose fermenter, no H₂S

• Campylobacter jejuni is the most common bacterial cause of gastroenteritis worldwide.

• Strongly linked to foodborne outbreaks from undercooked poultry and unpasteurized milk.

• Clinical features: bloody diarrhea, fever, abdominal cramps; can lead to complications like Guillain–Barré syndrome and reactive arthritis.

• Other options:

  • Salmonella typhi → causes typhoid fever, spread via contaminated water/food but not specifically poultry.

  • Shigella dysenteriae → spread via fecal–oral route, not poultry-related.

  • Vibrio cholerae → causes cholera, linked to contaminated water/seafood.

1. Salt tolerance test:

   • Aeromonas hydrophila grows in 0% NaCl (freshwater organism).

   • Vibrio spp. (e.g., V. cholerae, V. parahaemolyticus) require salt (halophilic) and fail to grow in 0% NaCl.

2. Other differentiating traits:

   • Both are oxidase-positive (a) not useful).

   • Both can be motile (d) not useful).

   • Sucrose fermentation (c) is variable in both genera.

Helicobacter pylori is rapid urease-positive, producing urease that hydrolyzes urea into ammonia and CO₂, raising the pH (detected by color change in CLO test or urea broth within minutes to hours).

Why Not the Others?

• b) Proteus mirabilis: Urease-positive but slower (hours to days).

• c) Klebsiella pneumoniae: Variable (some strains are urease-positive, but not rapid).

• d) Escherichia coli: Urease-negative.

Yersinia pestis is the bacterium responsible for plague, a severe and historically devastating disease. It manifests in three clinical forms:

1. Bubonic plague (swollen lymph nodes/buboes).

2. Septicemic plague (bloodstream infection).

3. Pneumonic plague (highly contagious respiratory form).

Why Not the Others?

• a) Cholera – Caused by Vibrio cholerae (watery diarrhea).

• c) Typhoid fever – Caused by Salmonella Typhi (systemic fever, abdominal pain).

• d) Dysentery – Caused by Shigella spp. or Entamoeba histolytica (bloody diarrhea).

Serratia marcescens is a Gram-negative bacillus known for:

1. DNase and lipase production:

   • DNase activity: Cleaves DNA (detected by DNase agar with methyl green).

   • Lipase activity: Hydrolyzes fats (e.g., positive on spirit blue agar).

2. Hospital outbreaks: Linked to contaminated IV fluids, catheters, and respiratory equipment.

3. Pigmentation: Some strains produce red prodigiosin (at 25°C).

Why Not the Others?

• a) Pseudomonas aeruginosa: Produces elastase/phospholipase (not classic DNase/lipase).

• c) Enterobacter cloacae: Lacks consistent DNase/lipase activity.

• d) Acinetobacter baumannii: Rarely produces DNase/lipase; associated with ventilators.

1. Primary transmission:

   • Consumption of raw/undercooked shellfish (especially oysters).

   • Exposure of wounds to seawater (e.g., fishermen, swimmers).

2. Clinical syndromes:

   • Severe sepsis (in immunocompromised patients, especially with liver disease).

   • Necrotizing wound infections.

3. Lab ID:

   • Halophilic (requires salt; grows on TCBS agar as green colonies).

   • Lactose-positive (unlike V. cholerae).

Why Not the Others?

• (a) Contaminated water: More typical of V. cholerae (causes cholera).

• (c) Airborne droplets: Irrelevant for Vibrio (spread via ingestion/trauma).

• (d) Soil contact: Associated with Clostridium or Bacillus anthracis.

Pseudomonas aeruginosa is a leading cause of nosocomial (hospital-acquired) pneumonia, particularly in cystic fibrosis (CF) patients. Its adaptability and resistance mechanisms make it a persistent pathogen in this vulnerable population.

Why Not the Others?

• b) Escherichia coli – More associated with UTIs or gastrointestinal infections.

• c) Klebsiella pneumoniae – Causes pneumonia but is less linked to CF.

• d) Proteus mirabilis – Primarily linked to UTIs and catheter-associated infections.

Helicobacter pylori is the primary pathogen linked to chronic gastritis, peptic ulcers, and even gastric cancer. It thrives in the acidic stomach environment by producing urease, which neutralizes gastric acid.

Why Not the Others?

• b) Vibrio cholerae – Causes watery diarrhea (cholera), not ulcers.

• c) Salmonella typhi – Causes typhoid fever (systemic infection).

• d) Shigella dysenteriae – Causes bloody diarrhea (dysentery).

1. Non-motile – Shigella species lack flagella (unlike E. coli, Enterobacter, and Proteus).

2. Non-lactose fermenting (NLF) – Pale colonies on MacConkey agar (though S. sonnei may show delayed lactose fermentation).

3. Clinical relevance: Causes dysentery (bloody diarrhea with fever).

Why Not the Others?

• (a) Escherichia coli:

  • Motile (peritrichous flagella) and lactose fermenting (pink colonies on MacConkey).

• (c) Enterobacter cloacae:

  • Motile and lactose fermenting (similar to E. coli).

• (d) Proteus vulgaris:

  • Highly motile (swarming on blood agar) and non-lactose fermenting, but H₂S-positive (blackening in TSI).

• Bordetella pertussis is the causative agent of whooping cough (pertussis), characterized by severe paroxysmal coughing and inspiratory “whoop.”

• Key virulence factors: pertussis toxin, filamentous hemagglutinin (FHA), and adenylate cyclase toxin.

• Diagnosed via PCR or nasopharyngeal swab culture on Bordet-Gengou or Regan-Lowe agar.

Why Not the Others?

• (b) Haemophilus ducreyi → Causes chancroid (genital ulcers).

• (c) Mycoplasma pneumoniae → Causes atypical (“walking”) pneumonia.

• (d) Legionella pneumophila → Causes Legionnaires’ disease (pneumonia).

Proteus mirabilis is well known for its swarming motility on blood agar, producing concentric waves of growth that spread across the surface of the agar. This is due to its highly active flagella and ability to undergo rapid motility.

• b) Pseudomonas aeruginosa – Motile but does not swarm; it may show spreading or mucoid growth.

• c) Salmonella typhi – Motile but does not exhibit swarming.

• d) Shigella sonnei – Non-motile (no swarming).

Vibrio cholerae is the causative agent of cholera, characterized by profuse “rice-water stools”—watery diarrhea with flecks of mucus, resembling water used to rinse rice.

Key Features of Cholera:

1. Toxin-mediated: Cholera toxin (CT) causes severe electrolyte/water loss via activation of adenylate cyclase in intestinal cells.

2. Transmission: Contaminated water/food (e.g., seafood).

3. Rapid dehydration: Can be fatal within hours if untreated.

Diagnosis & Treatment:

• Stool microscopy (curved, motile rods on dark-field).

• Culture on TCBS agar (yellow colonies).

• Oral rehydration therapy (ORT) + antibiotics (doxycycline/azithromycin).

Why Not the Others?

• b) E. coli: Causes traveler’s diarrhea (not rice-water stools).

• c) Shigella sonnei: Causes dysentery (bloody/mucoid stools).

• d) Salmonella enterica: Causes enterocolitis (non-watery diarrhea).

The oxidase test is the most reliable method to distinguish Pseudomonas aeruginosa (oxidase-positive) from Enterobacteriaceae (oxidase-negative).

Key Differences:

1. Oxidase Test:

   • P. aeruginosa: Positive (produces cytochrome c oxidase).

   • Enterobacteriaceae (e.g., E. coli, Klebsiella, Proteus): Negative.

2. Other Tests:

   • Lactose fermentation (a): P. aeruginosa is a non-fermenter, but so are some Enterobacteriaceae (e.g., Salmonella).

   • Urease (c) and indole (d): Variable in both groups (not definitive).

1. “Gull-wing” appearance:

   • Seen under microscopy (Gram stain or wet mount) due to its spiral or S-shaped morphology with polar flagella, resembling seagull wings.

2. Additional traits:

   • Gram-negative, motile, and oxidase-positive.

   • Causes bacterial gastroenteritis (diarrhea, fever, abdominal pain).

3. Culture requirements:

   • Grows under microaerophilic conditions (5% O₂, 10% CO₂) at 42°C (selective in stool cultures).

Why Not the Others?

• (a) Vibrio cholerae: Comma-shaped (no gull-wing), oxidase-positive.

• (c) Helicobacter pylori: Spiral-shaped but associated with gastric ulcers (no gull-wing term).

• (d) Legionella pneumophila: Rod-shaped, no distinctive gull-wing morphology.

• Escherichia coli → Indole-positive

• Enterobacter cloacae → Indole-negative

• Other points:

  • Both are Gram-negative, lactose-fermenting, oxidase-negative Enterobacteriaceae.

  • Citrate utilization → Enterobacter is usually citrate-positive, but some E. coli can also use citrate weakly, so indole is the more reliable differentiator.

  • Oxidase → both are oxidase-negative.

  • Urease → both usually urease-negative (except some Enterobacter strains).

The Triple Sugar Iron (TSI) agar results described are:

• Red slant/yellow butt: Indicates alkaline slant (no sugar fermentation in slant) and acidic butt (glucose fermentation).

• No gas or H₂S: No bubble or cracking in the agar (no gas production) and no blackening (no H₂S).

This pattern is typical of non-lactose/non-sucrose fermenters that only ferment glucose.

Breakdown of options:

• Shigella sonnei (Correct):

  • Ferments glucose but not lactose/sucrose → Red slant/yellow butt.

  • No H₂S or gas production → Matches the description.

• Escherichia coli:

  • Ferments glucose, lactose, and sucrose → Yellow slant/yellow butt.

  • Often produces gas (cracks in agar).

• Salmonella enterica:

  • Ferments glucose but not lactose/sucrose → Red slant/yellow butt.

  • Produces H₂S (black precipitate) and often gas.

• Proteus vulgaris:

  • Ferments glucose but not lactose/sucrose → Red slant/yellow butt.

  • Produces H₂S (blackening) and often gas.

Most Salmonella species are hydrogen sulfide (H₂S) producers, which can be observed as blackening on Triple Sugar Iron (TSI) agar or Salmonella-Shigella (SS) agar.

• a) Lactose non-fermenters – Salmonella does not ferment lactose (unlike E. coli).

• c) Indole-negative – Salmonella is typically indole-negative (unlike E. coli).

• d) Oxidase-negative – Salmonella is oxidase-negative (unlike Pseudomonas).

• Vibrio parahaemolyticus is a non-lactose fermenting, halophilic Gram-negative bacillus.

• Associated with gastroenteritis after eating raw or undercooked shellfish (especially oysters).

• Causes watery diarrhea, abdominal cramps, nausea, sometimes fever.

• Other options:

  • Salmonella enterica → gastroenteritis (often poultry/eggs, not shellfish).

  • Shigella dysenteriae → dysentery, person-to-person spread, not shellfish.

  • E. coli → many diarrheal syndromes, but typically linked to beef, contaminated produce, not shellfish.

Legionella pneumophila is a fastidious Gram-negative bacterium that requires Buffered Charcoal Yeast Extract (BCYE) agar for optimal growth. This specialized medium provides:

• L-cysteine and iron, which are essential nutrients for Legionella.

• Charcoal to detoxify free radicals.

• Buffer to maintain a pH suitable for growth.

Why Not the Others?

• a) Blood agar – Lacks L-cysteine and iron; Legionella does not grow.

• b) Chocolate agar – Enriched but missing key supplements (e.g., L-cysteine).

• d) MacConkey agar – Inhibits Legionella (does not ferment lactose)

• Campylobacter jejuni is a curved, motile, Gram-negative bacillus.

• It grows optimally at 42 °C (which corresponds to the body temperature of birds, its natural reservoir).

• It is the leading cause of bacterial gastroenteritis worldwide, often linked to undercooked poultry and contaminated food/water.

• Clinical presentation: fever, abdominal cramps, and bloody diarrhea (inflammatory).

• Other options:

  • Vibrio cholerae → causes rice-water stools, grows best on TCBS agar, not specifically at 42 °C.

  • Escherichia coli → some pathotypes cause diarrhea, but not 42 °C growth.

  • Klebsiella pneumoniae → associated with pneumonia, not gastroenteritis.

Klebsiella pneumoniae, particularly carbapenem-resistant K. pneumoniae (CRKP), is a major multidrug-resistant (MDR) pathogen notorious for resistance to carbapenems (e.g., meropenem, imipenem) and other last-line antibiotics.

Key Traits of CRKP:

1. Resistance Mechanism: Produces carbapenemases (e.g., KPC, NDM, OXA-48 enzymes) that hydrolyze carbapenems.

2. Hospital-Acquired Infections: Pneumonia, UTIs, bloodstream infections.

3. Treatment Challenges: Limited to polymyxins (colistin), tigecycline, or newer β-lactam/β-lactamase inhibitors (e.g., ceftazidime-avibactam).

Why Not the Others?

• a) Escherichia coli: Some strains are carbapenem-resistant (e.g., CRE), but K. pneumoniae is more commonly associated with outbreaks.

• c) Pseudomonas aeruginosa: Often MDR but typically resists carbapenems via porin mutations (not carbapenemases).

• d) Proteus mirabilis: Rarely carbapenem-resistant; more associated with UTIs and urease production.

Yersinia pestis, the causative agent of plague, exhibits a distinctive “safety-pin” appearance on bipolar staining (Wright-Giemsa or Wayson stain) due to its bipolar uptake of dye.

Key Traits of Y. pestis:

1. Gram-negative coccobacillus with rounded ends.

2. Transmission: Flea bites (Xenopsylla cheopis) or respiratory droplets (pneumonic plague).

3. Clinical Forms:

   • Bubonic plague (swollen lymph nodes/buboes).

   • Septicemic plague (bloodstream infection).

   • Pneumonic plague (highly contagious, fatal if untreated).

Why Not the Others?

• a) Salmonella Typhi: Rod-shaped, no bipolar staining.

• c) Vibrio cholerae: Curved rods (“comma-shaped”).

• d) Shigella sonnei: No bipolar staining.

Yersinia enterocolitica infections are most commonly linked to contaminated pork (e.g., undercooked pork products like chitterlings or raw pork intestines). The bacterium thrives in pigs, which serve as a major reservoir.

Key Features of Y. enterocolitica Infection:

1. Clinical Presentation:

   • Enterocolitis (fever, diarrhea, abdominal pain mimicking appendicitis).

   • Post-infectious complications (e.g., reactive arthritis).

2. Cold-Tolerant: Can grow in refrigerated foods (4°C).

Why Not the Others?

• a) Undercooked beef: More associated with E. coli O157:H7 or Salmonella.

• c) Seafood: Linked to Vibrio or Norovirus.

• d) Dairy products: Associated with Listeria or Campylobacter.

The indole test is the primary test to differentiate:

• Proteus mirabilis: Indole-negative.

• Proteus vulgaris: Indole-positive.

Supporting Tests:

1. Citrate utilization (b):

   • P. mirabilis: Positive (can grow on Simmons citrate agar).

   • P. vulgaris: Negative.

2. Urease (c): Both are urease-positive (not useful for differentiation).

3. Oxidase (d): Both are oxidase-negative.

Pseudomonas aeruginosa is:

• Oxidase-positive (produces cytochrome c oxidase, a key differentiating feature from Enterobacteriaceae).

• A non-fermenter (metabolizes glucose oxidatively via the O-F test, not fermentatively).

Why Not the Others?

• a) Escherichia coli – Oxidase-negative, ferments glucose.

• c) Klebsiella pneumoniae – Oxidase-negative, ferments glucose.

• d) Proteus mirabilis – Oxidase-negative, ferments glucose.

1. Rapid urease activity (within minutes):

   • H. pylori produces abundant urease, hydrolyzing urea to ammonia and CO₂, causing a rapid pH shift (detected by CLOtest or urea broth turning pink/red).

2. Clinical relevance:

   • Causes chronic gastritis, peptic ulcers, and gastric cancer.

3. Other traits:

   • Curved/spiral Gram-negative rods, microaerophilic, and oxidase-positive.

Why Not the Others?

• (a) Proteus mirabilis: Urease-positive but takes hours (not minutes).

• (c) Klebsiella pneumoniae: Urease-positive but slow/sometimes variable.

• (d) Morganella morganii: Urease-positive but not as rapid as H. pylori.

Shigella species are non-motile, which helps differentiate them from other enteric bacteria like Salmonella or E. coli (which are motile).

Why not the others?

• a) Motile – Incorrect (Shigella lacks flagella).

• b) Lactose fermenters – Most Shigella species do not ferment lactose, except S. sonnei (late fermenter).

• d) H₂S producers – Shigella is H₂S-negative (unlike Salmonella).

A key characteristic of Escherichia coli is that it is indole-positive, meaning it produces the enzyme tryptophanase, which breaks down tryptophan into indole (detected using Kovac’s reagent).

• a) Oxidase-negative – E. coli is oxidase-negative, not positive.

• c) Motile – Most E. coli strains are motile due to peritrichous flagella.

• d) Non-H2S producer – E. coli typically does not produce hydrogen sulfide (H₂S) on standard media like TSI or SIM.