- A gram-positive, catalase-positive, aerobic rod is isolated from a food sample implicated in an outbreak. A wet mount preparation reveals a characteristic 'tumbling' motility at 25°C. The isolate grows on bile esculin agar, producing a black discoloration. Which organism is described?
Answer: Listeria monocytogenes
This is a classic description of Listeria monocytogenes. It is a gram-positive rod, catalase-positive, and exhibits tumbling motility, which is most pronounced at room temperature (25°C). A key biochemical feature is its ability to hydrolyze esculin in the presence of bile, a test often associated with Enterococcus. Bacillus cereus is motile but does not show tumbling motility and is typically bile esculin negative. Enterococcus is a catalase-negative coccus. Corynebacterium species are generally non-motile.
- A Christensen's urea agar slant is inoculated with a gram-negative rod. A bright, intense pink color develops throughout the slant and butt within 4 hours of incubation. The speed of this reaction is most suggestive of which of the following organisms?
Answer: Proteus vulgaris
The urease test detects the hydrolysis of urea into ammonia, which raises the pH and turns the phenol red indicator pink. While several organisms are urease-positive, the rate of the reaction is a key differential feature. Proteus species are 'rapid urease producers,' causing a dramatic color change within 1-6 hours. Klebsiella and some Serratia are urease-positive but are considered 'slow' producers, often requiring 24 hours or more for a positive result. E. coli is urease-negative.
- A motile, H₂S-positive, gram-negative rod is tested on Lysine Iron Agar (LIA). The result is a red slant over a yellow butt (R/A) with a black precipitate in the butt. This result allows for differentiation from which other common H₂S-positive enteric pathogen?
Answer: Salmonella enterica
The LIA result of R/A indicates a positive reaction for lysine deaminase (red slant) and a negative reaction for lysine decarboxylase (yellow butt). Lysine deaminase activity is characteristic of the Proteus, Providencia, and Morganella genera. This R/A reaction clearly differentiates the isolate from Salmonella enterica, which is also H₂S-positive and motile but is lysine deaminase-negative and lysine decarboxylase-positive, yielding a purple over purple (K/K) reaction on LIA.
- An alpha-hemolytic, catalase-negative, gram-positive coccus is isolated from sputum. When tested for optochin susceptibility using a 5 µg disk, a zone of inhibition of 12 mm is measured. The laboratory's protocol defines susceptibility as ≥14 mm and resistance as <14 mm for a 6mm disk. What is the most appropriate next step to identify this isolate?
Answer: Perform a bile solubility test as a confirmatory method for identification.
An optochin zone of 12 mm is considered equivocal or intermediate. It does not meet the criteria for susceptibility (≥14 mm) but is not definitively resistant. In such cases, a confirmatory test is required to differentiate S. pneumoniae from optochin-susceptible viridans streptococci. The bile solubility test is the standard confirmatory test; S. pneumoniae is soluble in bile, whereas viridans streptococci are not.
- An anaerobic gram-negative rod is tested for susceptibility to special potency disks to aid in identification. The organism shows resistance to a 5 µg vancomycin disk, a 1000 µg kanamycin disk, and a 10 µg colistin disk. It also demonstrates good growth on an agar medium containing 20% bile. This pattern is highly characteristic of which group of organisms?
Answer: Bacteroides fragilis group
The combination of bile tolerance (growth on 20% bile agar) and resistance to vancomycin, kanamycin, and colistin is the classic presumptive identification pattern for the Bacteroides fragilis group, the most common anaerobic pathogens. Other anaerobic gram-negative rods have different susceptibility patterns: Fusobacterium is susceptible to kanamycin and colistin; Prevotella and Porphyromonas are typically bile-sensitive and have variable susceptibility to the special potency disks.
- A microbiologist is identifying a motile, gram-negative rod from a wound infection. Key test results include a negative indole test, a positive methyl red test, a negative Voges-Proskauer test, and a K/A H2S-positive result on TSI agar. The organism is also rapidly urease-positive. Which of the following is the most likely identification?
Answer: Proteus mirabilis
This combination of results points strongly to Proteus mirabilis. It is motile, indole-negative (unlike P. vulgaris), MR-positive/VP-negative, and rapidly urease-positive. Its TSI reaction is typically K/A with H2S production. Salmonella has a similar TSI profile but is urease-negative. Shigella is non-motile, urease-negative, and H2S-negative. E. coli is typically indole-positive, urease-negative, and H2S-negative.
- A Gram stain from a positive blood culture reveals small, coccobacillary organisms that are difficult to decolorize, and are reported as 'gram-variable.' The organism grows on MacConkey agar. Further testing shows the isolate is oxidase-negative, catalase-positive, and strictly non-motile. Which organism best fits this complex description?
Answer: Acinetobacter baumannii
Acinetobacter baumannii is a notorious gram-negative coccobacillus that often resists decolorization and can appear gram-positive or gram-variable. Crucially, it grows on MacConkey agar (differentiating it from many fastidious organisms and gram-positives), is oxidase-negative (differentiating it from Pseudomonas), catalase-positive, and non-motile. H. influenzae does not grow on MacConkey. S. epidermidis is gram-positive and does not grow on MacConkey. P. aeruginosa is a motile rod and is oxidase-positive.
- A gram-negative rod is inoculated into a Sulfide-Indole-Motility (SIM) medium tube. After incubation, the entire tube is filled with a black precipitate, obscuring the stab line. When Kovac's reagent is added, a red ring forms at the surface. How should the motility for this organism be interpreted and reported from this tube?
Answer: Indeterminate, because the heavy H₂S production prevents a conclusive reading of motility.
In SIM medium, motility is determined by observing diffuse growth (turbidity) spreading away from the stab line. When an organism produces a large amount of hydrogen sulfide (H₂S), the resulting black precipitate can fill the tube, making it impossible to see the pattern of growth. While it is tempting to assume the organism must be motile to spread the precipitate, the H₂S gas can diffuse on its own. The correct procedure is to report the motility as indeterminate or unreadable and rely on an alternative method (e.g., wet mount) if needed.
- An isolate of gram-positive cocci in clusters from a blood culture is tested for coagulase. The slide coagulase test is negative. However, the tube coagulase test is positive after 4 hours of incubation at 37°C. Which statement best explains this discrepancy?
Answer: The organism produces extracellular free coagulase but lacks cell wall-bound coagulase (clumping factor).
The slide coagulase test detects bound coagulase, also known as clumping factor. The tube coagulase test detects free coagulase, an extracellular enzyme. Some strains of Staphylococcus aureus lack clumping factor but still produce free coagulase. Therefore, the tube test is considered definitive. A slide-negative, tube-positive result is characteristic of these strains of S. aureus.
- A beta-hemolytic, catalase-negative, gram-positive coccus is isolated. A CAMP test is performed by streaking the isolate perpendicular to a streak of beta-lysin-producing Staphylococcus aureus. An arrowhead-shaped zone of inhibited hemolysis is observed at the junction of the two streaks. This phenomenon, known as the reverse CAMP test, is characteristic of which organism?
Answer: Arcanobacterium haemolyticum
The standard CAMP test, which shows synergistic enhancement of hemolysis, is positive for Streptococcus agalactiae. The 'reverse' CAMP test described, where hemolysis is inhibited, is due to the phospholipase D produced by Arcanobacterium haemolyticum inhibiting the activity of the staphylococcal beta-lysin. S. pyogenes is CAMP-negative. Listeria gives a different 'block-type' positive CAMP reaction and is a gram-positive rod.
- An organism isolated from a stool culture of a patient with fever and abdominal pain is a gram-negative rod that produces non-lactose-fermenting colonies on MacConkey agar. Further testing reveals it is motile at 25°C but non-motile at 37°C. It is also urease-positive and ornithine decarboxylase (ODC)-positive. Which organism exhibits this unique set of characteristics?
Answer: Yersinia enterocolitica
The key differential characteristic described is temperature-dependent motility: motility at room temperature (25°C) but not at body temperature (37°C). This is a hallmark feature of Yersinia enterocolitica. The other biochemical results (NLF on MacConkey, urease-positive, ODC-positive) are also consistent with this identification. Salmonella is motile at both temperatures and urease-negative. Shigella is non-motile. Vibrio is motile but does not exhibit this specific temperature-dependent pattern.
- A nitrate broth is inoculated with an unknown bacterium. After incubation, nitrate reagents A and B are added, and no red color develops. Subsequently, a small amount of zinc dust is added, and the broth immediately turns red. What is the correct interpretation of this test?
Answer: The organism is unable to reduce nitrate, leaving it unchanged in the medium.
The nitrate reduction test is a two-step process. First, reagents are added to detect nitrite. No color change means either nitrate was not reduced, or it was reduced completely past nitrite. The addition of zinc dust is the second, confirmatory step. Zinc chemically reduces any remaining nitrate to nitrite. If the broth turns red after zinc is added, it proves that nitrate was present, meaning the organism did not reduce it. This is a true negative result for nitrate reduction by the bacterium.
- A Lysine Iron Agar (LIA) slant is inoculated with a gram-negative rod. After incubation, the slant is purple, the butt is purple, and there is no black precipitate. Which metabolic capabilities can be attributed to this organism based on this K/K result?
Answer: Glucose fermentation and lysine decarboxylation, but no lysine deamination.
A purple/purple (K/K) result on LIA indicates a specific sequence. First, the organism ferments the small amount of glucose in the butt, creating an acidic environment. This acid activates the lysine decarboxylase enzyme. Decarboxylation of lysine produces alkaline amines, which neutralize the acid and revert the pH indicator to purple. The purple slant indicates a negative result for lysine deaminase. Therefore, the organism ferments glucose and decarboxylates lysine.
- A gram-negative rod is found to be a non-lactose fermenter on MacConkey agar after 24 hours. However, a subsequent o-nitrophenyl-β-D-galactopyranoside (ONPG) test is positive. What does this combination of results indicate about the organism's lactose metabolism?
Answer: The organism possesses the enzyme β-galactosidase but lacks the permease enzyme to transport lactose into the cell.
Lactose fermentation requires two proteins: β-galactoside permease (to transport lactose into the cell) and β-galactosidase (to hydrolyze lactose into glucose and galactose). The ONPG test directly detects the presence of β-galactosidase. A positive ONPG test in an organism that appears as a non-lactose fermenter on primary media indicates that it possesses the necessary enzyme but lacks the transport protein (permease). These organisms are often called 'late' or 'slow' lactose fermenters.
- A lactose non-fermenting, gram-negative rod from a urine culture is rapidly urease-positive. A phenylalanine deaminase (PAD) test is performed by inoculating a slant, incubating, and adding 10% ferric chloride. A deep green color develops immediately. What is the significance of this result?
Answer: It indicates the organism belongs to the Proteus, Providencia, or Morganella group.
The PAD test detects the enzyme phenylalanine deaminase, which converts phenylalanine to phenylpyruvic acid. The addition of the ferric chloride reagent results in a visible green color in the presence of phenylpyruvic acid. A positive PAD test is a key characteristic used to identify and differentiate the genera Proteus, Providencia, and Morganella from other members of the Enterobacteriaceae. Klebsiella is PAD-negative.
- A gram-negative rod from a cystic fibrosis patient's sputum grows as non-lactose-fermenting colonies on MacConkey agar and produces a green, water-soluble pigment with a characteristic grape-like odor. An O/F glucose test shows acid production only in the open (aerobic) tube. The oxidase test is positive. Which organism is most likely?
Answer: Pseudomonas aeruginosa
The combination of pyocyanin (green pigment) production, grape-like odor, a positive oxidase test, and an oxidative (not fermentative) metabolism is classic for Pseudomonas aeruginosa. The other options are also non-fermenters found in CF patients, but they do not match the complete profile. Stenotrophomonas is oxidase-negative. Acinetobacter is oxidase-negative. Burkholderia is typically oxidase-positive but does not produce the characteristic pigment and odor described.
- An unknown gram-negative rod is inoculated onto a Triple Sugar Iron (TSI) agar slant. After 18 hours of incubation, the entire tube is yellow. After 24 hours, the slant has reverted to red, while the butt remains yellow with gas production and a black precipitate. How should this organism's carbohydrate metabolism be characterized?
Answer: It ferments glucose only, and the red slant at 24 hours is due to aerobic peptone catabolism producing alkaline byproducts.
The TSI agar contains glucose (0.1%), lactose (1%), and sucrose (1%). The initial yellow tube indicates fermentation of glucose, producing acid. Because the glucose concentration is low, it is exhausted quickly in the aerobic slant. The organism then begins to catabolize peptones aerobically, producing alkaline amines that raise the pH and cause the phenol red indicator in the slant to revert to red (K). In the anaerobic butt, glucose fermentation persists, so it remains yellow (A). Lactose/sucrose fermenters would produce enough acid to keep both slant and butt yellow (A/A). A non-fermenter would not produce a yellow butt (K/NC).
