01Core principlesThe concepts and mechanisms needed to understand the subject.
Medical microbiology asks whether a microorganism is present, viable, causative, transmissible and susceptible to feasible treatment. Bacteria are cellular organisms that usually reproduce by binary fission; many grow outside host cells, while obligate intracellular bacteria such as Chlamydia trachomatis require a host-cell environment. Classification includes cell envelope, morphology, metabolism and genetic traits. Viruses require host cells and may be detected through nucleic acid, antigen, culture in specialised systems or host response. Fungi range from yeasts to moulds with distinct tissue and immune patterns. Parasites have life cycles in which stage, host and geography determine the useful specimen and test.
The pre-analytical phase often determines value. A superficial swab may sample colonisers while missing a deep process; insufficient blood volume reduces culture sensitivity; delayed transport changes fragile organisms; prior antimicrobial exposure can sterilise culture. The request should state syndrome, site, onset, travel or exposure, immune status and treatment because laboratories select media, incubation, molecular targets and safety procedures from that context. Good sampling is a clinical intervention, not clerical preparation.
Laboratory methods answer different questions. Microscopy can rapidly reveal cells, organisms or inflammation but detection thresholds and observer skill limit a negative result. Culture proves growth under the conditions used and permits phenotypic susceptibility, yet slow, fastidious or treated organisms may not grow. PCR or other nucleic-acid amplification is sensitive and rapid for a specified target but can detect residual material and cannot always distinguish colonisation from invasion. Serology often demonstrates host exposure and depends on timing, baseline prevalence and paired change.
Interpretation integrates host and specimen. A common skin organism in one bottle may be contamination, but the same organism in multiple sets with an intravascular device and matching illness can be causal. Growth from a normally sterile compartment carries different weight from a colonised surface. Antimicrobial stewardship starts with timely effective treatment for serious suspected infection, then focuses: review at a defined interval, narrow, change route, stop or define duration using microbiology, diagnosis, source control, organ function and clinical response.
Key points
- Begin with syndrome, anatomical site, host and exposure; organism lists without these anchors generate poor specimens and misleading positives.
- Collect the right specimen from the infected compartment, in adequate volume, before antimicrobials when safe, using correct transport and clear clinical details.
- Microscopy is rapid but variably sensitive; culture recovers viable organisms; antigen and nucleic-acid tests detect targets that may persist without viable infection.
- Colonisation, contamination and infection are clinical–microbiological interpretations, not properties printed automatically beside an organism name.
- Susceptibility categories depend on organism, drug exposure, breakpoints and method; an in-vitro result does not guarantee penetration, safety or clinical response.
- In severe infection, obtain decisive samples without harmful treatment delay, then review empirical therapy against results, source control and trajectory.
- Communicate urgent or unexpected results directly, initiate appropriate infection-control and public-health action, and document ownership.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Meningitis, pneumonia, bloodstream infection and wound colonisation require different specimens and causal thresholds even when the same organism is detected.
Age, anatomical breach, immune defect, devices, recent healthcare and antimicrobials reshape the organism range and performance of diagnostic methods.
Airborne, droplet, contact, food, water, vector, sexual and bloodborne routes determine exposure history, isolation and public-health implications.
Oxygen requirement, intracellular lifestyle, temperature and nutritional needs explain culture conditions and why routine media can miss a pathogen.
Target change, drug inactivation, reduced permeability, efflux and pathway bypass can be intrinsic or acquired and may affect related agents together.
An organism at a non-sterile site becomes causal evidence through compatible inflammation, tissue access, burden, repeated detection and exclusion of alternatives.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Direct microscopy and staining - Why
- Provide rapid cellular and morphological evidence from an appropriate specimen.
- Interpretation and limitations
- Organism burden, specimen quality and observer skill affect sensitivity; morphology guides but rarely completes species-level identification.
- 02
Culture with identification - Why
- Recover viable organisms for identification and often phenotypic susceptibility testing.
- Interpretation and limitations
- Negative culture may follow prior treatment or unsuitable conditions, while positive non-sterile-site culture requires causal assessment.
- 03
Nucleic-acid amplification - Why
- Detect a defined microbial genetic target rapidly, including organisms difficult to culture.
- Interpretation and limitations
- A valid negative result means no included target was detected in that specimen. It does not exclude infection by a panel organism or an organism outside the panel; consider sampling, timing, target variation, inhibition and assay sensitivity alongside the clinical syndrome.
- 04
Antigen detection - Why
- Identify a microbial component in a selected specimen with faster turnaround than culture.
- Interpretation and limitations
- Sensitivity varies by burden and timing, and antigen persistence can outlast active replication; confirm or correlate according to the syndrome.
- 05
Serology - Why
- Detect host antibody suggesting recent, past or immune exposure when direct detection is unsuitable.
- Interpretation and limitations
- Ig class alone may not date infection; paired titres, avidity or confirmatory methods may be required and immunodeficiency can blunt response.
- 06
Antimicrobial susceptibility testing - Why
- Estimate whether achievable exposure is likely to inhibit the isolate under standardised conditions.
- Interpretation and limitations
- Apply current breakpoints and dosing assumptions, then integrate site penetration, allergy, organ function, source control and patient response.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked caseInterpret possible blood-culture contaminationInputs: one of four bottles grows coagulase-negative staphylococci at 38 hours; the patient is improving, has no intravascular device, and repeat cultures before further antibiotics remain negative.+
- 1Assess organism and compartment: coagulase-negative staphylococci can cause bloodstream infection but commonly enter cultures from skin during collection.
- 2Weight pattern and host evidence: one late-positive bottle, no device, improving physiology and negative repeat sets lower the probability of true sustained bacteraemia.
- 3Check sampling quality, exact species, susceptibility, collection sites and whether antibiotics preceded repeats before final interpretation.
- 4Conclude that contamination is more likely than bloodstream infection in this stated scenario, while retaining reassessment if fever, repeated concordant growth or a device emerges.
- 5Use antimicrobial review to avoid unnecessary continuation while ensuring the original clinical syndrome has another adequate explanation.
- 6Verify through trajectory and repeat evidence: continued recovery without concordant cultures supports contamination; deterioration or repeated same-organism growth reverses the judgement and prompts source evaluation.
02Specimen pathwayProtect diagnostic yield in severe infectionA patient has suspected sepsis and antimicrobials are about to start.+
- 1Stabilise immediately and identify likely anatomical source, host risks and required isolation.
- 2Obtain appropriate blood and source specimens promptly before treatment only when this does not create harmful delay.
- 3Communicate syndrome, site, timing, exposure and prior therapy to the laboratory and start current syndrome-appropriate empirical treatment.
- 4Review results, source control and physiology at a defined interval to narrow, stop, switch or escalate with microbiology advice.
03Molecular resultInterpret detection after treatmentA sensitive nucleic-acid test remains positive after symptoms improve and culture is negative.+
- 1Check what target the assay detects, specimen site, expected clearance kinetics and whether it distinguishes viability.
- 2Compare cycle or quantitative information only if the assay validates that clinical use; do not improvise infectiousness thresholds.
- 3Integrate symptoms, immune status, culture, inflammatory evidence and alternative diagnoses.
- 4Seek specialist or public-health advice when clearance testing affects isolation or treatment rather than extending therapy automatically.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Track preliminary, amended and final microbiology results to a named clinician and document direct communication of critical findings.
- At antimicrobial review, record diagnosis, microbiological evidence, source control, drug choice, route and intended duration or stop criterion.
- Repeat sampling only when persistence, treatment failure, clearance policy or a new syndrome creates a defined question.
- Monitor local resistance and outbreak signals because empirical choices and infection-control actions depend on current epidemiology.
- If laboratory and clinical trajectories disagree, recheck specimen identity, timing, target, prior treatment and whether more than one process is present.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
More sensitive can find less relevant
A molecular assay may detect low-level or residual targets that culture misses; analytical detection becomes useful only after clinical and compartment interpretation.
Volume is diagnostic power
For blood culture, insufficient sampled blood can matter more than adding exotic assays because intermittent low-level bacteraemia requires adequate opportunity for capture.
Breakpoints include exposure assumptions
Susceptibility categories are not purely organism labels; method, achievable concentration and sometimes dose or site inform their intended interpretation.
Source control changes microbiology
Draining infected material or removing an infected device can determine outcome even when the selected antimicrobial is active in vitro.
Negative panels have boundaries
A multiplex result covers named targets and validated specimens; it cannot exclude organisms absent from the panel or below detection after treatment.
07Common pitfallsFrequent interpretation and management errors.
- 01
Sending a superficial or low-volume specimen because it is easier, then treating its organisms as representatives of deep infection.
- 02
Delaying life-saving antimicrobial treatment in an unstable patient solely to obtain ideal specimens.
- 03
Equating nucleic-acid detection with viable, causal and transmissible infection in every context.
- 04
Calling a likely contaminant without checking number of positive sets, time to positivity, device status, host risk and clinical course.
- 05
Treating the susceptibility report without considering source, penetration, organ function, allergy, interaction and source control.
- 06
Starting broad empirical therapy without a documented review time, diagnostic goal or responsibility for focusing it.