Microbiology: MRCEM Primary questions with answers

Microbiology carries 17 of the 180 questions in MRCEM Primary, and this topic is our largest question set. It begins with the principles of microbiology, culture media and investigation, and the UK immunisation schedule. It then covers key bacteria, viruses, protozoa, fungi and worms, and notifiable diseases. Emergency physicians must recognise sepsis, meningitis and serious infection quickly, and know which diseases must be reported.

5 free questions below, each with the answer and the full explanation · 362 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · Respiratory Viruses

   A girl aged 4 is brought to the emergency room by her parents. She has been experiencing vomiting and watery diarrhea for the last 12 hours. Her parents mention that several children in her playgroup have also been sick recently after eating at a picnic. Which test is most appropriate to confirm whether she has norovirus?

   1. ART-PCR test
   2. BUrine test
   3. CStool culture
   4. DEnzyme immunoassay (EIA)
   5. EBlood test
   Show answer and explanation

   Answer: A. RT-qPCR is the preferred diagnostic method for norovirus.

   • B: Urine tests are not useful for norovirus.
   • C: Stool culture does not detect norovirus.
   • D: EIA is not recommended for single sporadic samples or in place of RT-qPCR.
   • E: Blood tests are not useful for norovirus.

   Full explanation

   • RT Q-PCR, or Reverse Transcription Quantitative Polymerase Chain Reaction assays are the preferred diagnostic method to detect norovirus.
   • Enzyme Immune Assay tests should not replace RT-qPCR during outbreak investigations; they are not recommended for testing single samples from sporadic cases.
   • Other tests like stool culture and blood tests are not useful for diagnosing norovirus.

2. Question 2 · Streptococcus and Staphylococcus Bactera

   A male aged 35 attends with fever and a cough productive of small amounts of clear sputum. Chest radiography shows bilateral lower lobe infiltrates that are more pronounced on the right side. What is the causative agent?

   1. AStreptococcus pnemoniae
   2. BMycoplasma pneumoniae
   3. CS. aureus
   4. DInfluenza virus
   5. EPseudomonas
   Show answer and explanation

   Answer: A. Fever, cough and lower lobe infiltrates fit pneumococcal pneumonia, a leading cause of community-acquired pneumonia.

   • B: Mycoplasma usually gives dry cough and extrapulmonary features.
   • C: S. aureus follows influenza or cavitation in specific settings.
   • D: Influenza virus usually causes viral pneumonia with diffuse features.
   • E: Pseudomonas typically affects bronchiectasis or hospitalised patients.

   Full explanation: Streptococcus and Staphylococcus Bactera

   Streptococcus spp

   Common features: 

   Gram stain	Gram positive
   Shape	Cocci Note: Cocci in chains: Streptococcus Pyogenes Cocci in clusters: Staph. Aureus and Staph Epidermidis Diplococci: Streptococcus pneumonia

   Streptococcus Spp

   • (Beta hemolytic group A) Is gram +Ve Cocci in chains  
   • Causes acute sore throat in children, skin and soft tissue infection and necrotizing fasciitis.  
   • Erysipelas: Superficial skin infection involving upper dermis and superficial lymphatics caused by Group A beta hemolytic streptococcus pyogenes 
   • Post-infectious immune-mediated reactions of Streptococcus Pyogenes include Rheumatic fever, erythema nodosum, acute glomerulonephritis. 

    

   Staphylococcus Epidermidis:

   • Gram positive cocci is an important pathogen involving prosthetic implants, intravascular lines, ventriculoperitoneal shunts and cardiac valves. 
   • Staphylococcus epidermidis is part of the normal flora of the skin and nasopharynx, and infection is usually endogenous. 

    

   Streptococcus Viridans 

   Microorganism	Streptococcus Viridans
   Gram stain	Gram positive
   Shape	Cocci (chains)
   Disease	Endocarditis after tooth extraction Dental abscess

    

   Streptococcus Pneumoniae:

   Gram positive Alpha hemolytic encapsulated diplococci transmitted via respiratory droplets causing pneumonia, Meningitis, sinusitis and otitis media 

   Community acquired pneumonia: 

   • Investigation  
   • Blood tests- FBC, CRP U&E 
   • CXR 
   • A definitive diagnosis of CAP requires evidence of consolidation on chest x-ray. 
   • Pulse oximetry ABGs 

   Blood & Sputum culture (± Gram stain) – ideally before antibiotics are given, in all patients with moderate- or high-severity CAP (as determined by the CURB-65 score) presenting in hospital. 

   Severity assessment and management (NICE & BTS guideline) 

    

    

   Choice of antibiotics 

   Treatment	Antibiotic, dosage and course length
   First-choice oral antibiotic if low severity	Amoxicillin: 500 mg TDS for 5 days
   If penicillin allergic/alternative or atypical pathogen suspected then:	Doxycycline: 200 mg on first day, then 100 mg OD for 4 days. Clarithromycin:500 mg BD a day for 5 days Erythromycin (in pregnancy): 500 mg four times a day for 5 days.
   First-choice oral antibiotics if moderate severity	Amoxicillin: 500 mg TDS for 5 days With (if atypical pathogens suspected) Clarithromycin:500 mg BD a day for 5 days OR Erythromycin (in pregnancy): 500 mg four times a day for 5 days.
   Alternative oral antibiotics if moderate severity, for penicillin allergy:	Doxycycline: 200 mg on first day, then 100 mg OD for 4 days. Clarithromycin:500 mg BD a day for 5 days
   First-choice antibiotics if high severity	Co-amoxiclav: 500/125 mg TDS a day orally or 1.2 g TDS intravenously for 5 days With Clarithromycin: 500 mg BD orally or intravenously a day for 5 days OR Erythromycin (in pregnancy): 500 mg four times orally a day for 5 days.
   Alternative antibiotic if high severity, for penicillin allergy:	Levofloxacin (consider safety issues):500 mg BD orally or intravenously for 5 days

    

   Staphylococcus aureus:

   Staphylococcus aureus is a catalase-positive, coagulase-positive, beta-haemolytic, Gram-positive coccus found in clusters. 

   Transmission:  

   Endogenous spread: Staphylococcus aureus is normal gut flora of skin and nasopharynx. Endogenous spread causes infection in immunocompromised individuals 

   Exogenous spread: via direct skin contact or indirect contact by clothing or other fomites 

   • Disease: Primary skin infection (e.g., impetigo, abscess and cellulitis) 
   • Secondary skin infections (e.g., in eczema, surgical wounds, intravenous devices) 
   • Pneumonia  
   • Infective Endocarditis (most common cause of acute IE in IV drug users 
   • Osteomyelitis 
   • Septic arthritis 

    

   Septic Arthritis:

   • is an infection of any kind that finds its way into joint space, it is a most rapid and destructive joint disease, acute onset of monoarticular arthritis with pain swelling, and warmth, the knee is the most common site others include the hip, wrist, shoulder, and ankle. 
   • The most common type of bacteria that causes septic arthritis is called staphylococcus aureus. others include streptococcus, gram-negative rods, 
   • Neisseria Gonorrhea causes septic arthritis in sexually active young adults. 

   Treatment: Flucloxacillin: Antibiotic of choice 

   • Vancomycin is used to treat methicillin Resistant Staph Aureus 

    

3. Question 3 · Pseudomonas Aeruginosa, Legionella Pneumophilia and Bortonella Henslae-Bacteria

   A nursing home resident aged 77 is brought into the ED pyrexial with shortness of breath and purulent sputum. She is diagnosed with pneumonia.

   Which pathogen is most in keeping with an endogenous source of infection?

   1. APseudomonas aeruginosa
   2. BMycoplasma pneumonia
   3. CPneumocystis Jiroveci
   4. DLegionella pneumophila
   5. EChlamydophila psittaci
   Show answer and explanation

   Answer: A. Pseudomonas aeruginosa is an opportunistic pathogen and colonises the airways of nursing home residents (endogenous source).

   • B: Mycoplasma spreads by droplets from others.
   • C: Pneumocystis affects immunocompromised patients.
   • D: Legionella is acquired from water sources.
   • E: Chlamydophila psittaci is acquired from birds.

   Full explanation: Pseudomonas Aeruginosa, Legionella Pneumophilia and Bortonella Henslae-Bacteria

   Pseudomonas Aeruginosa

   • Gram-negative rod transmitted via direct/indirect contact likely in the hospital environment.

    Legionella Pneumophilia

   • Gram-negative coccobacilli are transmitted via respiratory aerosol. 

   Legionnaires disease 

   • Diagnosis: Legionella antigen detection in urine 
   • Treatment: Quinolones like levofloxacin or macrolides like clarithromycin 

   Pseudomonas aeruginosa	Gram negative rod causes Nosocomial, or hospital acquired or nursing home Infection (mainly pneumonia, UTI and wound Infection)
   Legionella pneumophila	Gram negative coccobacilli Reservoir: air-conditioning units High risk factors include older people, alcoholic and immunosuppression Causing pneumonia Note: Hyponatremia is important laboratory finding occurs more common in Legionella pneumophila

     

   Bartonella Henselae

   • Gram negative bacilli or coccobacilli  
   • Cat-scratch disease (CSD) or felinosis is an infectious disease caused by Bartonella henselae most often results from a scratch or bite of a cat.   

4. Question 4 · Escherichia Coli, Klebsiella, Salmonella-Bacteria

   A male aged 15 years, a known case of sickle cell disease with a history of splenectomy 1 year back, presented to the ED with bloody diarrhea. What is the most likely causative agent?

   1. ANeisseria meningitides
   2. BStaph aureus
   3. CPseudomonas aeruginosa
   4. DHemophilus influenza
   5. EE-Coli
   Show answer and explanation

   Answer: E. E. coli is a recognised cause of bloody diarrhoea or gastroenteritis in post-splenectomy patients.

   • A: Neisseria meningitidis causes sepsis and meningitis in asplenia, not bloody diarrhoea.
   • B: Staph aureus causes food poisoning with toxin, not bloody diarrhoea.
   • C: Pseudomonas does not typically cause bloody diarrhoea.
   • D: Haemophilus influenzae causes invasive encapsulated infection, not bloody diarrhoea.

   Full explanation: Escherichia Coli, Klebsiella, Salmonella-Bacteria

   Escherichia Coli:

   Note: E. coli serotype O157 causes bloody diarrhoea associated with hemolytic uremic syndrome in children, characterized by hemolytic anemia, thrombocytopenia and acute renal failure. 

    

   Klebsiella spp: 

   Salmonella enterica

   • is a gram-negative rod that causes enterocolitis (Food poisoning), Enteric fever (Typhoid), Septicemia, and osteomyelitis 
   • Primarily transmitted through fecal-oral route usually from ingestion of contaminated poultry, eggs, and uncooked meat 

   
   

   Treatment:  

   Salmonella enterocolitis: Usually supportive management in resistant cases ciprofloxacin 

   Enteric fever: First line ciprofloxacin if organism sensitive, and in resistant cases cefotaxime or azithromycin 

   High yield: 

   Regarding osteomyelitis 

   • Most common cause generally: Staph Aureus 
   • The most common cause in IV drug abuser: Staph Aureus 
   • The most common cause in sickle cell disease: Salmonella 
   • Most common cause after hip replacement surgery: Staph epidermidis 

     

5. Question 5 · Notifiable Disease

   Which one of the following diseases is not notifiable in the UK?

   1. AChicken pox
   2. BSmall pox
   3. CMeasles
   4. DMalaria
   5. ECovid-19
   Show answer and explanation

   Answer: A. Chickenpox is not notifiable in the UK, unlike the other listed diseases.

   • B: Smallpox is notifiable.
   • C: Measles is notifiable.
   • D: Malaria is notifiable.
   • E: Covid-19 is notifiable.

   Full explanation: Notifiable Disease

   Diseases notifiable to local authority proper officers under the Health Protection (Notification) Regulations 2010:

   • Acute encephalitis
   •  Acute infectious hepatitis
   • Acute meningitis
   • Acute poliomyelitis
   • Anthrax
   • Botulism
   • Brucellosis
   • Cholera
   • COVID-19
   • Diphtheria
   • Enteric fever (typhoid or paratyphoid fever)
   • Food poisoning
   • Haemolytic uremic syndrome (HUS)
   • Infectious bloody diarrhea
   • Invasive group A streptococcal disease
   • Legionnaires’ disease
   • Leprosy
   • Malaria
   • Measles
   • Meningococcal septicaemia
   •  Monkeypox
   • Mumps
   • Plague
   • Rabies
   • Rubella
   • Severe Acute Respiratory Syndrome (SARS)
   • Scarlet fever
   • Smallpox
   • Tetanus
   • Tuberculosis
   • Typhus
   • Viral hemorrhagic fever (VHF)
   • Whooping cough
   •  Yellow fever