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Microbiology

Applied Basic Sciences · 18 questions · 2 free to try

The organisms behind surgical infection and how they are prevented and treated: surgical site infection, Clostridioides difficile, MRSA and resistant Gram negatives, necrotising infections, blood-borne viruses, tetanus and the prophylaxis a surgeon must prescribe correctly.

Every question in this topic is a single best answer item in the style of the exam, followed by an explanation that gives the reasoning for the correct option, why each distractor is wrong, a key point to carry into the exam and, where one applies, the guideline or trial it rests on. Practice mode lets you work through the topic on its own or mixed with others; exam mode draws it into timed papers.

Sample questions from this topic

A theatre list includes a thyroidectomy, an elective cholecystectomy, a right hemicolectomy and an appendicectomy for a perforated gangrenous appendix with pus. The infection control lead asks the registrar to classify these wounds and state the expected infection rates. Which account is correct?

  1. AAll surgical wounds carry much the same postoperative infection risk once appropriate antibiotic prophylaxis has been given, whatever the wound classification says
  2. BA perforated appendix with pus is classified as a 'clean-contaminated' wound
  3. CProphylactic antibiotics convert a dirty wound into a clean one
  4. DClean (thyroidectomy, ~1 to 2%), clean-contaminated (elective cholecystectomy), contaminated (spillage), dirty (established pus — the perforated appendix)
  5. EClean wounds never become infected, so antibiotic prophylaxis is always unnecessary even for implant surgery
Show answer and explanation

Correct answer: D

The wound classification is surgery's oldest risk model and still frames antibiotic and closure decisions. Clean: no entry into respiratory, alimentary, genitourinary tracts, no inflammation, asepsis unbroken — thyroidectomy, hernia repair; baseline infection 1 to 2% (rising with patient factors — the diabetes, smoking, obesity themes of the SSI bundle — and justifying prophylaxis only when a prosthesis makes rare infection catastrophic, per the mesh question). Clean-contaminated: a hollow viscus opened in a controlled fashion without unusual spillage — elective cholecystectomy, prepared colorectal resection, gastric and gynaecological surgery; around 5 to 10%, the category where timed antibiotic prophylaxis earns most of its living. Contaminated: gross gastrointestinal spillage, entry through acutely inflamed non-purulent tissue, major breaks in technique, and fresh (

Why the others are wrong: A, B, C and E — risk is class-dependent; pus makes a wound dirty by definition; antibiotics reduce but never reclassify risk; clean wounds still infect at low rates, and prosthetic implants justify prophylaxis there.

Key point: Clean (~1–2%), clean-contaminated (~5–10%), contaminated (~15–20%), dirty (~30–40%): the class predicts SSI, decides prophylaxis (single timed dose) versus treatment (continued course), and steers closure strategy — and a case can be upgraded by what happens on the table.

Guidelines: CDC/NHSN surgical wound classification, applied in NICE surgical site infection guidance (NG125): clean, clean-contaminated, contaminated and dirty classes predict infection risk and determine antibiotic prophylaxis versus treatment.

Blood cultures from a patient with cholangitis grow a Gram-negative rod, an intra-abdominal abscess grows a mixed anaerobic culture, and a cellulitic leg yields a Gram-positive coccus in chains. The microbiology round asks which organisms dominate which surgical infections. Which account is correct?

  1. ABiliary sepsis: enteric Gram-negatives with enterococci; abscesses: mixed faecal flora with Bacteroides; skin: staphylococci and streptococci
  2. BA single organism causes all surgical infections
  3. CBacteroides fragilis is an aerobic skin commensal
  4. DCellulitis of the lower leg is usually caused by anaerobic Gram-negative rods translocating across the bowel wall from the patient's own gut flora
  5. EEnterococci never appear in biliary infection
Show answer and explanation

Correct answer: A

Empirical antibiotics are educated guesses, and the education is this map. Biliary tract: an obstructed duct breeds enteric organisms — Escherichia coli and Klebsiella dominate cholangitis and cholecystitis blood cultures, with Enterococcus a regular companion (relevant because cephalosporins miss it) and anaerobes joining after biliary-enteric anastomoses or stents: the piperacillin-tazobactam or cephalosporin-plus-metronidazole logic of the cholangitis question, always paired with the decompression that actually treats it. Colonic flora: the distal gut's ~10¹² organisms per gram are mostly anaerobes — Bacteroides fragilis the great abscess-former, its polysaccharide capsule orchestrating the walled-off collections of diverticular and appendiceal disease — mixed with coliforms and streptococci (Streptococcus anginosus/milleri group flagging abscesses anywhere): faecal peritonitis therefore always needs anaerobic cover (metronidazole) with Gram-negative cover, the composition behind every 'perforation' regimen in this bank. Skin and soft tissue: Staphylococcus aureus (coagulase-positive, abscess- and toxin-forming — wound infections, boils, line sepsis, and the MRSA variant with its decolonisation pathway) and the beta-haemolytic streptococci — Streptococcus pyogenes (group A) causing spreading cellulitis and erysipelas (dermal, sharply demarcated) and the type II monomicrobial necrotising fasciitis of the pain-out-of-proportion question, whose toxin production clindamycin suppresses. Hospital-acquired and critical-illness infection shifts right: Pseudomonas aeruginosa, ESBL and carbapenemase producers, enterococci and Candida (the upper-GI-perforation fungaemia lesson) — why ICU empirical regimens broaden and why cultures-then-de-escalation is stewardship's engine.

Why the others are wrong: B, C, D and E — flora is site-specific; B. fragilis is a gut anaerobe; cellulitis is streptococcal/staphylococcal; enterococci are habitual biliary residents.

Key point: Biliary = coliforms + enterococcus; colonic = coliforms + Bacteroides (metronidazole always); skin = Staph. aureus and group A strep (nec fasc's soloist); ICU = Pseudomonas, resistant Gram-negatives, Candida — empirical therapy is anatomy-guessed microbiology, corrected by cultures.

Guidelines: UKHSA Start Smart Then Focus: empirical antibiotic choice is built on the predictable flora of each site — enteric Gram-negatives and anaerobes for biliary and intra-abdominal sepsis, beta-haemolytic streptococci for cellulitis — refined by local policy and cultures.

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