MLT GlobeBacteriology: Culture media, specimens & AST – page 4
126 Bacteriology MCQs on Culture media, specimens & AST with answers and explanations.
Which of these is an enriched special medium rather than a basic medium?
Bordet-Gengou contains potato, glycerol and blood for Bordetella pertussis. Peptone water, nutrient broth and meat extract agar are simple basal media.
Which organism is intrinsically resistant to vancomycin?
Vancomycin is too large to cross the Gram-negative outer membrane, so Enterobacterales are inherently resistant. Oral vancomycin is a treatment for C. difficile; MSSA and S. pyogenes are susceptible.
A semisolid medium for checking bacterial motility should contain agar at roughly which concentration?
About 0.4–0.5% agar keeps the medium soft enough for flagellated bacteria to spread from the stab line. About 1.5% is standard solid agar; much higher concentrations (about 5–6%) are used to inhibit Proteus swarming.
In the pour-plate method for viable counts, the diluted inoculum is:
In pour plates the sample is mixed with cooled molten agar, so colonies grow both within and on the medium. Spreading on set agar is the spread-plate method.
Stuart's (or Amies) transport medium is especially important for swabs that may contain:
Gonococci die quickly from drying and cooling; non-nutritive Stuart or Amies medium keeps them viable. Cary–Blair is used for enteric pathogens and vibrios.
Which incubation condition is required to recover Bacteroides fragilis from an abscess?
B. fragilis is an obligate anaerobe; it is grown anaerobically, often on BBE agar. Microaerobic 42 °C incubation is used for Campylobacter.
In industrial penicillin production, Penicillium chrysogenum was classically grown in a medium based on:
Corn steep liquor supplies nitrogen and phenylacetic acid precursors that raised penicillin yields dramatically.
Drops of serial dilutions are placed on an agar surface and colonies in the drop areas counted to estimate CFU/mL. This is the:
The Miles and Misra surface-drop technique estimates viable counts. ELISA and Widal detect antigen or antibody.
Blood culture bottles contain which anticoagulant, chosen because it also inactivates complement and some antibiotics?
SPS (0.025–0.05%) prevents clotting and neutralises complement, lysozyme and aminoglycosides. Citrate and EDTA can inhibit bacteria.
A colleague says plasmid-borne antimicrobial resistance is a problem only in Gram-negative bacteria. Which statement about plasmid-mediated resistance is actually FALSE?
Gram-positive bacteria also carry resistance plasmids (e.g. blaZ penicillinase plasmids in Staphylococcus aureus, vanA-bearing plasmids in enterococci). The other statements are true of plasmid-mediated resistance.
Urinalysis shows many pus cells, yet the routine aerobic urine culture yields no growth. Which of the following is LEAST likely to explain this finding?
Sterile pyuria reflects inflammation without growth on routine media: mycobacteria, chlamydiae and prior antibiotics all cause it. Suprapubic aspiration is simply a contamination-free collection method, not a cause of pyuria.
Which practice gives the best chance of detecting bacteremia in an adult?
Yield rises with the volume of blood cultured and the number of sets. Several sets also help tell true infection from skin contamination. Antibiotics given first reduce recovery.
An E. coli resistant to ceftazidime and cefotaxime shows a much larger zone when clavulanic acid is added to the disk. This indicates:
ESBLs break down third-generation cephalosporins but are inhibited by clavulanate, so the combination disk zone grows (about 5 mm or more). AmpC enzymes are not inhibited by clavulanate.
A Gram stain of an expectorated sputum shows more than 25 squamous epithelial cells and fewer than 10 neutrophils per low-power field. What is the correct action?
Many squamous cells with few neutrophils show the sample is mostly saliva from the mouth. Culture would grow oral flora and mislead, so the specimen should be rejected and recollected.
Four blood culture bottles (two sets) are collected from a patient with fever. Only one aerobic bottle grows coagulase-negative staphylococci after 3 days. This result most likely represents:
Coagulase-negative staphylococci in a single bottle out of several sets usually come from skin. True bacteraemia normally gives growth in more than one set, often with a shorter time to positivity.
A wound specimen is plated on phenylethyl alcohol (PEA) agar. The main purpose of this medium is to:
PEA inhibits gram-negative bacteria, including swarming Proteus, so gram-positive cocci and many anaerobes can be isolated from mixed specimens. It contains no lactose or H2S indicator.
On Hektoen enteric agar, a stool isolate forms blue-green colonies with black centres. This appearance is most typical of:
Salmonella does not ferment the sugars in HE agar (blue-green colony) and produces H2S (black centre). Shigella is blue-green without black centres, and E. coli ferments lactose giving orange-salmon colonies.
All disk diffusion QC zones are too large on a new batch of Mueller-Hinton agar plates. Which cause is most likely?
The standard agar depth is about 4 mm; thin agar lets the drug spread further, giving larger zones and false susceptibility. A heavy inoculum gives smaller zones, and 35 °C in air is the correct incubation.
A technologist accidentally uses a 2.0 McFarland suspension instead of 0.5 McFarland for disk diffusion. What effect is expected?
A heavier inoculum reaches visible density before the drug has diffused far, so zones are smaller and results can appear falsely resistant. A too-light inoculum causes the opposite error.
Routine Mueller-Hinton disk diffusion plates for Enterobacterales are incubated in ambient air rather than 5% CO2 because CO2:
CO2 forms carbonic acid, lowering the pH; this reduces the activity of aminoglycosides and macrolides and increases that of some other drugs. Incubation in CO2 is used only for organisms that need it, with specific breakpoints.