Microbiology can feel like a collection of unrelated names, laboratory tests and disease associations. A more effective approach to MRCEM Primary microbiology revision is to organise organisms into groups, understand what distinguishes them and practise applying those differences to single best answer questions.
This guide covers bacterial identification, mechanisms of infection, viruses, fungi, parasites and antimicrobial principles, followed by five original practice SBAs with explanations. Use it alongside the syllabus for your examination sitting and our complete MRCEM Primary revision guide.
What should you revise for MRCEM Primary microbiology?
For each organism, ask: What type of organism is it? How is it transmitted? What helps it cause disease? Which features distinguish it from the alternatives? This creates a usable framework rather than a list of disconnected facts.
Principles of microbiology versus pathogen groups
Principles explain how microorganisms behave: cell structure, replication, transmission, host defence, virulence and antimicrobial resistance. Pathogen groups apply those principles to particular bacteria, viruses, fungi and parasites.
Both matter. Recognising a bacterium from its appearance is useful, but understanding its structure may also explain why a particular drug target is absent. Build revision notes around comparisons, then test whether you can explain the differences without looking at your notes.
Which syllabus applies to your examination?
The older RCEM MRCEM Primary information pack describes 180 SBAs over three hours and allocates 17 questions to microbiology. That allocation belongs to the blueprint used for the October 2026 examination.
RCEM announced on 3 August 2026 that an updated Basic Sciences Syllabus will apply from the 21 April 2027 examination onwards. Microbiology remains one of the six named disciplines. Do not assume that the older question allocation or detailed topic list is unchanged for later sittings: check RCEM’s syllabus announcement and the current MRCEM examination information.
Build a practical framework for identifying bacteria
Start with Gram reaction, then shape and arrangement, then distinguishing laboratory or clinical features. “Gram-positive cocci” narrows the possibilities; “in clusters, catalase positive and coagulase positive” narrows them much further.
The following tables describe classic teaching patterns. Real laboratory identification combines methods, and atypical isolates occur. In an SBA, use all the supplied information rather than treating one feature as infallible.
Gram-positive cocci: separating similar organisms
OrganismTypical appearanceUseful distinguishing featureExample association
Staphylococcus aureus | Gram-positive cocci in clusters | Catalase positive; usually coagulase positive | Abscesses, bacteraemia and endocarditis
Staphylococcus epidermidis | Gram-positive cocci in clusters | Coagulase negative; biofilm formation | Prosthetic-device infection
Staphylococcus saprophyticus | Gram-positive cocci in clusters | Coagulase negative; classically novobiocin resistant | Urinary tract infection
Streptococcus pyogenes | Gram-positive cocci in chains | Group A; beta-haemolytic | Pharyngitis, cellulitis and invasive infection
Streptococcus agalactiae | Gram-positive cocci in chains | Group B; usually beta-haemolytic | Neonatal infection
Streptococcus pneumoniae | Gram-positive, often lancet-shaped diplococci | Alpha-haemolytic; classically optochin sensitive and bile soluble | Pneumonia and meningitis
Enterococcus species | Gram-positive cocci in pairs or chains | Bile-esculin positive; commonly PYR positive | Urinary infection and endocarditis
Catalase breaks down hydrogen peroxide and helps distinguish staphylococci from streptococci and enterococci. Coagulase provides a further classic distinction between S. aureus and common coagulase-negative staphylococci. Haemolysis describes the appearance around colonies on blood agar: alpha indicates partial haemolysis and beta indicates complete haemolysis.
Do not equate “coagulase negative” with “harmless”. Skin organisms may contaminate samples, but they can also cause important infections involving intravascular lines, prosthetic valves or joints.
Gram-negative cocci and bacilli
OrganismTypical appearanceUseful revision clueExample association
Neisseria meningitidis | Gram-negative diplococci | Oxidase positive; encapsulated invasive strains | Meningitis and meningococcal sepsis
Neisseria gonorrhoeae | Gram-negative diplococci | May be seen within neutrophils in appropriate specimens | Gonorrhoea and disseminated infection
Escherichia coli | Gram-negative bacillus | Usually lactose fermenting | Urinary infection and bacteraemia
Klebsiella pneumoniae | Gram-negative bacillus | Prominent capsule; lactose fermenting | Pneumonia and urinary infection
Pseudomonas aeruginosa | Gram-negative bacillus | Oxidase positive; non-fermenter | Opportunistic and healthcare-associated infection
Haemophilus influenzae | Small Gram-negative coccobacillus | Requires X and V factors for growth | Respiratory and invasive infection
A coccobacillus is a short rod. The oxidase test detects activity associated with cytochrome c oxidase. Lactose fermentation is another useful discriminator, but interpret it alongside the organism’s other properties rather than in isolation.
Acid-fast organisms, atypical bacteria and anaerobes
Mycobacteria have lipid-rich cell walls containing mycolic acids and are identified using acid-fast methods. Mycoplasma lacks a peptidoglycan cell wall. Chlamydia is an obligate intracellular bacterium. These distinctions explain why routine Gram-stain patterns are insufficient for every organism.
Anaerobes grow without oxygen; obligate anaerobes cannot grow in its presence. Examples include Bacteroides fragilis and several clinically important clostridial organisms. Oxygen requirement, spore formation and Gram reaction describe separate properties: “anaerobic” does not automatically mean “spore forming”.
Understand mechanisms of infection
Connect microbial structure to function. Ask whether a feature helps a pathogen attach, invade, survive host defences or damage tissues.
Capsules, spores and virulence factors
Capsules can protect bacteria against phagocytosis. Pili and other adhesins help attachment to host surfaces. Biofilms allow organisms to persist within communities attached to surfaces, including medical devices. Endospores are resistant survival structures rather than a method of reproduction.
FeatureMain conceptCommon confusion
Capsule | Evasion of host defence | A capsule is not the same as a cell wall
Adhesin | Attachment to host cells | Attachment and toxin production are separate mechanisms
Biofilm | Persistence on a surface | Biofilm-associated tolerance is not identical to inherited antibiotic resistance
Endospore | Survival under adverse conditions | Sporulation does not increase bacterial numbers
Exotoxins and endotoxin
FeatureExotoxinsEndotoxin
Nature | Proteins produced by certain bacteria | Lipid A-containing component of Gram-negative outer membranes
Organisms | Can be produced by Gram-positive or Gram-negative bacteria | Associated with Gram-negative bacteria
Effects | Often specific effects on cells, signalling or neurotransmission | Activates inflammatory responses and can contribute to shock
Examples | Tetanus toxin, botulinum toxin and cholera toxin | Lipid A within lipopolysaccharide or lipooligosaccharide
Avoid the shortcut “exotoxin equals Gram positive”: cholera toxin is produced by a Gram-negative organism. Endotoxin can be released during growth as well as bacterial breakdown. See the NCBI overview of bacterial pathogenesis for further explanation.
Colonisation, contamination and infection
Colonisation means an organism is present without causing disease at that site. Contamination means an organism has entered a specimen during collection or processing. Infection involves invasion or multiplication in the host; clinical disease depends on the interaction between organism and host, and infection can sometimes be asymptomatic.
A positive result therefore needs context. Coagulase-negative staphylococci in one blood-culture bottle may represent contamination. Repeated positive cultures in a patient with a prosthetic valve warrant a different interpretation. Neither the organism’s name nor the result alone settles the question.
Organise viruses, fungi and parasites without endless lists
Use structure, transmission and host factors as starting points. Add detail when it explains a distinction you repeatedly miss.
Viral structure, transmission and replication
Virus or groupStructural featureRevision connection
Herpesviruses | Enveloped, double-stranded DNA viruses | Latency and reactivation
Adenoviruses | Non-enveloped, double-stranded DNA viruses | Respiratory, ocular and gastrointestinal disease
Influenza viruses | Enveloped viruses with segmented RNA genomes | Genetic change and reassortment
Norovirus | Non-enveloped RNA virus | Gastroenteritis and outbreaks
HIV | Enveloped RNA retrovirus | Reverse transcription and integration
For influenza, distinguish antigenic drift, involving accumulated mutations, from antigenic shift, an abrupt major antigenic change in influenza A. Reassortment involves exchange of genome segments when compatible influenza viruses infect the same cell. The CDC explanation of drift and shift explains the distinction.
An envelope is a useful structural feature, but does not determine clinical behaviour by itself. Learn transmission routes and disease associations separately rather than assuming every virus in a structural group behaves identically.
Fungi and opportunistic infection
OrganismUseful featureRelevant presentation
Candida species | Yeasts; some species also form pseudohyphae or hyphae | Mucosal disease or invasive infection
Aspergillus species | Moulds with septate hyphae | Invasive disease in susceptible immunocompromised patients
Cryptococcus species | Encapsulated yeasts | Pulmonary or central nervous system infection
Pneumocystis jirovecii | A fungus despite its historically confusing classification | Pneumonia in patients with impaired immunity
Host factors help distinguish these infections. Subacute breathlessness, dry cough and hypoxaemia in someone with advanced HIV should prompt consideration of Pneumocystis pneumonia. These clues support a differential; they are not diagnostic alone. The CDC provides overviews of fungal diseases and Pneumocystis pneumonia.
Parasites and exposure clues
Separate protozoa from helminths. Plasmodium is a protozoan transmitted by infected Anopheles mosquitoes. Giardia is an intestinal protozoan associated with faecal–oral transmission, including contaminated water. Schistosoma species are helminths acquired through skin exposure to contaminated freshwater in endemic areas.
For each parasite, learn transmission, relevant geographical exposure and the main site of disease. Avoid treating “returned traveller” as a diagnosis: several infections cause fever or diarrhoea, and timing and exposure details matter.
Connect microbiology to antimicrobial principles
Microbiology explains the organism; pharmacology explains how a drug interacts with it. Connecting the two helps you reason through questions rather than memorise drug lists without context.
Match antimicrobial targets to microbial structures
Target or processExample drug classCore mechanism
Cell-wall synthesis | Beta-lactams | Bind penicillin-binding proteins and inhibit peptidoglycan cross-linking
30S ribosomal subunit | Tetracyclines | Interfere with aminoacyl-tRNA binding
50S ribosomal subunit | Macrolides | Inhibit bacterial protein synthesis
DNA gyrase and topoisomerase IV | Fluoroquinolones | Interfere with bacterial DNA replication
Folate metabolism | Sulfonamides and trimethoprim | Inhibit different steps in folate synthesis
Mycoplasma provides a useful example: a drug acting on peptidoglycan synthesis lacks that target in an organism without a cell wall. This is a mechanism-based explanation, not a prescribing rule. Clinical antimicrobial selection also depends on the infection site, susceptibility results and current local guidance.
Understand resistance rather than memorising acronyms
Bacteria can resist antibiotics by destroying the drug, changing its target, reducing entry, increasing efflux or bypassing the affected pathway. Beta-lactamases illustrate drug destruction; altered penicillin-binding proteins illustrate target modification. These remain different mechanisms even when both reduce susceptibility to a beta-lactam antibiotic.
Resistance genes may spread between bacteria, and antibiotic exposure can select resistant populations. The CDC antimicrobial-resistance overview explains these mechanisms. Always connect a resistance label to the biological change it represents.
Test yourself with five original microbiology SBAs
These are original educational examples, not recalled questions or official RCEM examination material. Choose one best answer for each before reading the explanations.
Questions: identify the decisive clue
Question 1. A blood-culture isolate consists of Gram-positive cocci in clusters. It is catalase positive and coagulase positive. Which organism is the most likely identification?
- A. Staphylococcus epidermidis
- B. Staphylococcus aureus
- C. Streptococcus pyogenes
- D. Enterococcus faecalis
- E. Streptococcus pneumoniae
Question 2. A bacterial strain becomes substantially more susceptible to phagocytosis after losing its polysaccharide capsule. Which function of the capsule best explains the original strain’s greater virulence?
- A. Production of ATP
- B. Peptidoglycan cross-linking
- C. Protein synthesis
- D. Protection against host phagocytic clearance
- E. Formation of bacterial endospores
Question 3. Two influenza A viruses infect the same cell. A progeny virus contains genome segments derived from both parental viruses. Which process has occurred?
- A. Reassortment
- B. Conjugation
- C. Transformation
- D. Binary fission
- E. Antigenic drift due to point mutations alone
Question 4. A Staphylococcus aureus isolate carries mecA and produces an altered penicillin-binding protein with reduced affinity for many beta-lactam antibiotics. Which resistance mechanism is described?
- A. Enzymatic destruction of the antibiotic
- B. Increased antibiotic efflux
- C. Modification of the antibiotic target
- D. Reduced antibiotic entry through outer-membrane porins
- E. Bypass of folate synthesis inhibition
Question 5. A patient with advanced HIV presents with several weeks of progressive breathlessness, dry cough and hypoxaemia. Investigation confirms Pneumocystis jirovecii pneumonia. How is this organism classified?
- A. Gram-positive bacterium
- B. Gram-negative bacterium
- C. Protozoan
- D. Virus
- E. Fungus
Answers: why the alternatives are less appropriate
Question 1: B — Staphylococcus aureus. Clusters, catalase positivity and coagulase positivity support S. aureus. S. epidermidis is typically coagulase negative. S. pyogenes and E. faecalis are usually catalase negative and arranged in chains or pairs. S. pneumoniae is also catalase negative and characteristically forms diplococci. Learning point: use successive features to narrow identification.
Question 2: D — Protection against host phagocytic clearance. Susceptibility to phagocytosis is the decisive clue. ATP production is a metabolic function, peptidoglycan cross-linking concerns the cell wall, protein synthesis occurs at ribosomes, and endospore formation is a separate survival process. Learning point: link each bacterial structure to its function.
Question 3: A — Reassortment. The defining feature is exchange of whole genome segments. Conjugation and transformation are mechanisms of bacterial genetic exchange; binary fission is bacterial division. Antigenic drift involves accumulated mutations rather than exchange of intact segments. Reassortment can produce major antigenic change, but not every reassortment event necessarily does so. Learning point: distinguish a genetic mechanism from its possible antigenic consequence.
Question 4: C — Modification of the antibiotic target. mecA encodes the altered penicillin-binding protein PBP2a. Beta-lactamase production would describe drug destruction, while efflux pumps remove drugs and reduced permeability limits entry. S. aureus also lacks the Gram-negative outer membrane referenced in option D. Folate-pathway bypass is unrelated to the specified target. Learning point: methicillin resistance and beta-lactamase production are not interchangeable explanations.
Question 5: E — Fungus. Pneumocystis jirovecii is a fungus, although older terminology can cause confusion. It is neither a Gram-positive nor a Gram-negative bacterium, is not a virus and is no longer classified as a protozoan. Learning point: retain the modern classification alongside the familiar clinical syndrome.
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Turn mistakes into a focused microbiology revision plan
Use short question sets to expose weaknesses, then read selectively to correct them. Completing more questions is useful only when it changes what you understand or remember.
Classify your errors
Record whether a mistake came from missing knowledge, confusion between organisms or misreading the stem. Write one corrective sentence: “I confused coagulase with catalase” is more useful than “revise staphylococci”.
If you repeatedly confuse two organisms, build a table showing their shared features and the properties that separate them. Test yourself by covering the distinguishing-feature column rather than repeatedly reading the completed table.
Revisit weak concepts and then test them in context
Try a short topic-based question set, focused reading, recall without notes and a later mixed set. Revisit the concept after a few days and again the following week, adjusting the interval according to retention.
Eventually combine microbiology with pharmacology, pathology and other subjects. Mixed practice tests whether you recognise the relevant principle without being told the topic first. For the wider timetable, return to our complete MRCEM Primary revision guide.
Frequently asked questions
How many microbiology questions are in MRCEM Primary?
The older RCEM blueprint allocates 17 of 180 questions to microbiology. This applies to the blueprint used for October 2026. An updated syllabus becomes the official reference from 21 April 2027, so candidates for later sittings should verify the applicable blueprint rather than assuming the allocation remains unchanged.
Should I revise microbiology and pharmacology together?
Yes, where concepts overlap. Microbial structures, antimicrobial targets and resistance mechanisms fit naturally together. Keep separate objectives for organism identification and drug knowledge so that studying one does not leave gaps in the other.
Are question banks enough for microbiology revision?
Question banks identify weaknesses and develop SBA reasoning, but explanations and focused reading remain important when a concept is unclear. Aim to explain why an answer is correct and why the alternatives are less appropriate, rather than recognising a remembered question.
Where can I try MRCEM Primary practice questions?
StudyMedical offers a free MRCEM Primary demo with no sign-up required. It provides a straightforward way to sample questions before choosing a subscription.
Put your microbiology knowledge into practice
StudyMedical’s MRCEM Primary question bank offers over 7,800 questions, explanations with teaching diagrams, topic-based custom quizzes and timed practice. You can focus on new questions or revisit previously incorrect answers, with progress tracking to guide revision.
Explore the StudyMedical MRCEM Primary question bank and choose the plan that suits your preparation. Mock-paper access varies by plan; check the current inclusions before subscribing.
