Public Health Lab Quiz
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Public Health Microbiology Lab Quiz: Outbreak Investigation, Surveillance, Environmental Monitoring, and Epidemiological Typing
In a hospital clinical lab, a microbiologist identifies what is infecting one patient. In a public health lab, the question is different: what is infecting a community, how is it spreading, and how do we stop it? The work is less about individual diagnosis and more about patterns: spotting a cluster of cases that suggests a common source, identifying the same pathogen in water samples and in sick patients, or tracking the spread of an antibiotic-resistant organism across a region. Public health microbiology sits at the intersection of laboratory science and epidemiology, and it requires both skills.
This quiz is designed for public health laboratory scientists, environmental health officers, epidemiologists, microbiology students interested in population-level infectious disease work, and laboratory professionals preparing for roles in national or regional public health agencies. The questions cover outbreak investigation methodology, microbiological surveillance systems, environmental sampling and monitoring, the typing methods used to link cases to common sources, and the regulatory frameworks that govern public health laboratory work.
Core Topics
Outbreak Investigation: Identifying a Common Source
An outbreak is defined as an occurrence of cases of disease in a population that exceeds what would normally be expected for a given time and place. When an outbreak is suspected, the public health laboratory plays a central role in confirming the diagnosis, characterising the pathogen, and providing evidence that links cases to a common source or confirms person-to-person spread.
The Epidemiological Curve (Epi Curve) plots the number of new cases over time. The shape of the epi curve is one of the first clues to the nature of the outbreak. A point source outbreak, where all cases are exposed to the same source at roughly the same time (for example, a contaminated meal at a banquet), produces a sharp bell-shaped curve with most cases appearing within one incubation period of each other. A propagated outbreak, where the disease spreads from person to person, shows a stepped pattern with successive waves of cases over multiple incubation periods. A continuous common source outbreak, where exposure to the source is ongoing (for example, a contaminated water supply), produces a plateau-shaped curve that remains elevated as long as the source continues.
The case definition is a standardised set of clinical and epidemiological criteria used to determine who counts as a case in an outbreak investigation. A sensitive case definition (broad criteria) captures more cases, including mild or atypical ones, and is used in the initial phase of an investigation to ensure no cases are missed. A specific case definition (narrow criteria) is used when confirmation is needed and the objective shifts from case-finding to source identification.
Molecular Typing Methods for Public Health
Identifying the same pathogen in multiple cases is not sufficient to confirm a common-source outbreak. The isolates from all the cases need to be shown to be genetically related to each other and, if possible, to the suspected source. Molecular typing methods achieve this.
Pulsed-field gel electrophoresis (PFGE) was the gold standard for bacterial typing for several decades. PFGE uses restriction enzymes that cut bacterial DNA infrequently, producing large DNA fragments that are separated by a specialised gel electrophoresis technique using pulsing electric fields. The banding pattern of fragments is the bacterial DNA fingerprint. Isolates from an outbreak should share identical or very closely related PFGE patterns.
Whole-genome sequencing (WGS) is now rapidly replacing PFGE in high-income public health laboratories. WGS provides far more discriminatory power than PFGE, can identify the specific mutations responsible for antibiotic resistance, and allows phylogenetic analysis to determine evolutionary relationships between isolates. The COVID-19 pandemic dramatically accelerated the adoption of WGS in public health genomics globally. Multilocus sequence typing (MLST) characterises bacteria by the DNA sequences of several conserved housekeeping genes and assigns isolates to sequence types (STs), enabling comparison of isolates from different laboratories worldwide using a shared, publicly accessible database.
Environmental Monitoring and Sampling
Environmental sampling in public health microbiology involves testing water, food, air, and surfaces for the presence of indicator organisms and specific pathogens. Water microbiology monitoring relies primarily on indicator organisms rather than testing for every possible pathogen. The most widely used indicator organisms are total coliforms and E. coli. The presence of coliforms in drinking water indicates that the water treatment process has failed or that contamination has occurred after treatment. E. coli specifically indicates faecal contamination, suggesting that pathogens capable of causing gastrointestinal illness may be present.
The most probable number (MPN) method and membrane filtration (MF) method are the two principal techniques for quantifying coliforms and E. coli in water samples. The MPN method uses serial dilutions of the water sample inoculated into multiple wells of a growth medium and uses statistical tables to estimate the most probable number of organisms per 100 mL. The MF method filters a known volume of water through a membrane filter that retains bacteria, then incubates the filter on selective agar and counts colonies.
Notifiable Diseases and Reporting Requirements
Public health surveillance depends on clinicians and laboratories reporting cases of certain diseases to health authorities. Notifiable diseases are conditions that, when diagnosed, must be reported to local, national, or international health authorities within defined timeframes. Different countries have different lists of notifiable diseases, but most include: typhoid, cholera, tuberculosis, foodborne outbreaks, measles, viral haemorrhagic fevers, rabies, plague, and certain sexually transmitted infections. Laboratories often have direct reporting obligations in addition to (or instead of) clinician reporting, particularly for confirmed laboratory diagnoses of notifiable pathogens.
At the international level, the International Health Regulations (IHR 2005) require WHO member states to notify the WHO of events that may constitute a Public Health Emergency of International Concern (PHEIC). COVID-19 was declared a PHEIC in January 2020.
Frequently Asked Questions
What is an epi curve and how is it used?
An epidemiological curve (epi curve) is a bar chart showing the number of new outbreak cases plotted against time. The shape and spread of the curve helps determine the likely mode of transmission. A sharp peak spanning about one incubation period suggests a point source outbreak (a single shared exposure). A series of waves or steps suggests person-to-person spread (propagated outbreak). A plateau sustained over time suggests ongoing continuous exposure to a contaminated source.
What is PFGE and why is it used in outbreak investigation?
Pulsed-field gel electrophoresis (PFGE) is a molecular typing technique that produces a DNA fingerprint of a bacterial isolate. Bacterial DNA is cut with a restriction enzyme into large fragments, which are separated by gel electrophoresis using alternating electric fields. Isolates from a common-source outbreak should produce identical or closely related PFGE patterns. PFGE was the standard method for foodborne outbreak investigation for decades and is still used, though whole-genome sequencing is increasingly the preferred method.
What is the most probable number (MPN) method?
The MPN method is a statistical technique used to estimate the number of viable microorganisms in a water or food sample. The sample is serially diluted and each dilution is inoculated into multiple replicate tubes of a growth medium. After incubation, the presence or absence of growth is recorded at each dilution. A statistical table is used to estimate the most probable number of organisms per unit volume based on the pattern of positive and negative results. The MPN method is used widely in water quality testing for coliforms and E. coli.
What are indicator organisms in water quality testing?
Indicator organisms are microorganisms whose presence in water indicates contamination with faecal material and the likely presence of pathogens, without testing for every possible pathogen directly. Total coliforms are gram-negative bacteria from the Enterobacteriaceae family and their presence indicates water treatment failure or post-treatment contamination. E. coli is the preferred faecal indicator because it is specific to the gut of warm-blooded animals. Its presence in drinking water provides strong evidence of faecal contamination. Enterococcus is another widely used faecal indicator, particularly for recreational water quality assessment.
What is whole-genome sequencing (WGS) and how does it improve outbreak investigations?
Whole-genome sequencing determines the complete DNA sequence of a pathogen’s genome. In outbreak investigation, WGS provides far higher resolution than traditional typing methods, allowing investigators to determine with much greater confidence whether isolates from different patients are genetically related (part of the same outbreak) or are merely the same species. WGS also reveals antibiotic resistance genes, virulence factors, and evolutionary relationships between strains. During the COVID-19 pandemic, WGS was used globally to track the emergence and spread of new variants.
What is a notifiable disease?
A notifiable disease is a condition that, when diagnosed, must be reported to public health authorities within a defined timeframe. The list of notifiable diseases varies by country but typically includes serious communicable diseases including tuberculosis, typhoid, cholera, measles, viral haemorrhagic fevers, plague, and foodborne illness outbreaks. Mandatory reporting enables public health authorities to monitor disease trends, detect outbreaks, and implement control measures promptly. Laboratories often have direct reporting obligations separate from clinicians.
How does multilocus sequence typing (MLST) work?
MLST characterises bacteria by sequencing 7 conserved housekeeping genes. Each unique sequence at each locus is assigned an allele number. The combination of 7 allele numbers is the allelic profile, which is assigned a sequence type (ST) number. STs are registered in publicly accessible online databases, allowing comparison of isolates from laboratories in different countries. MLST is used to study the global epidemiology of bacterial pathogens including Staphylococcus aureus, Streptococcus pneumoniae, and Neisseria meningitidis.
What is the difference between endemic, epidemic, and pandemic?
Endemic refers to the constant baseline level of a disease in a specific geographic area or population (for example, malaria is endemic in sub-Saharan Africa). An epidemic is a sudden increase in the number of cases of a disease in a specific population above the expected baseline, in a defined area and time period. A pandemic is an epidemic that has spread across multiple countries or continents, typically affecting a large proportion of the world’s population. COVID-19 was declared a pandemic by the WHO in March 2020.
What is a PHEIC?
A Public Health Emergency of International Concern (PHEIC) is a formal declaration by the WHO under the International Health Regulations (IHR 2005) that an extraordinary public health event has occurred which poses a risk to other countries through international spread and requires a coordinated international response. PHEICs declared to date include the 2009 H1N1 influenza pandemic, the 2014 Ebola outbreak in West Africa, the Zika virus epidemic in 2016, and COVID-19 in 2020.
What is the membrane filtration method for water testing?
The membrane filtration (MF) method passes a known volume of water through a membrane filter (typically with a 0.45 micrometre pore size) that retains bacteria while allowing water to pass through. The filter is then placed on selective agar and incubated. Bacterial colonies that grow on the filter can be counted and characterised. For total coliforms and E. coli, specific chromogenic agars are used that produce coloured colonies allowing direct enumeration of each indicator organism. Results are expressed as colony forming units per 100 mL (CFU/100 mL).
What is HACCP and how does it relate to public health microbiology?
HACCP (Hazard Analysis and Critical Control Points) is a systematic, preventive approach to food safety that identifies physical, chemical, and microbiological hazards at each step of food production and processing, and establishes critical control points (CCPs) where controls can be applied to prevent or reduce these hazards to acceptable levels. It is a cornerstone of food safety management systems globally and is required by food safety regulators in most countries. Public health microbiologists interact with HACCP systems during food processing facility inspections, outbreak investigations, and verification testing.