Shortanswer — Pathogens, Prions, Rapid Methods, MPN and Virus Structure: 22-Agric-A7 Chemistry and Microbiology of Foods, May 2017, Q11

Question 11: Shortanswer — Pathogens, Prions, Rapid Methods, MPN and Virus Structure (16.7 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Human pathogens associated with bivalve molluscs and crustaceans

Filter-feeding bivalve molluscs (oysters, clams, mussels) concentrate whatever is present in their surrounding water, and crustaceans harvested from contaminated waters or handled unhygienically carry a similar risk profile. Key pathogens: Vibrio parahaemolyticus and Vibrio vulnificus (naturally occurring in warm coastal waters, the latter capable of severe, sometimes fatal septicaemia), Vibrio cholerae, norovirus (the leading cause of raw-oyster-associated outbreaks), and hepatitis A virus.

(b) Prions and their mode of disease

Prions are infectious misfolded proteins with no associated nucleic acid genome — a radical departure from every other infectious agent. The disease-causing form (PrP$^{Sc}$) is a misfolded conformer of a normal host membrane protein (PrP$^{C}$); disease propagates when PrP$^{Sc}$ contacts normal PrP$^{C}$ and templates its refolding into the same misfolded, protease-resistant, aggregation-prone conformation — a self-propagating chain reaction rather than replication in the conventional sense. The resulting PrP$^{Sc}$ aggregates accumulate in neural tissue, causing progressive, invariably fatal spongiform neurodegeneration.

(c) Four rapid microbiological methods

ATP bioluminescence — luciferin/luciferase light output proportional to microbial ATP, used for rapid hygiene/sanitation verification.
PCR/quantitative PCR — amplification and detection of pathogen-specific DNA sequences, giving same-day presumptive results versus days for culture.
ELISA (enzyme-linked immunosorbent assay) — antibody-based detection of a target pathogen's surface antigens or toxins.
Impedance/conductance microbiology — automated monitoring of the electrical-impedance change a growing culture produces in its medium, giving an automatically-detected presumptive growth/no-growth result far faster than manual colony counting.

(d) Principle of the Most Probable Number (MPN) technique

MPN is a statistical estimation method, not a direct count: a sample is serially diluted and multiple replicate tubes of growth medium are inoculated at each dilution level. After incubation, each tube is scored simply positive (growth) or negative (no growth). The observed pattern of positive/negative tubes across the dilution series is compared against a standard statistical (Poisson-distribution-based) probability table, which returns the bacterial density most likely to produce that particular pattern of positives and negatives — hence "most probable number" rather than an exact count.

(e) Enveloped vs. non-enveloped virus structure

i) Non-enveloped virus: an icosahedral protein capsid directly enclosing the nucleic-acid genome, exposed at the surface — typically more environmentally resistant (e.g. norovirus, hepatitis A). ii) Enveloped virus: the same capsid/genome core additionally wrapped in a host-cell-membrane-derived lipid envelope studded with glycoprotein spikes used for host-cell attachment; the envelope is fragile and easily disrupted by drying, detergents and gastric conditions.

Concept & Theory

Topic: Seafood-borne pathogens, prion biology, rapid microbiological methods, MPN statistics, and viral structure.

Key relations: template-directed protein misfolding (PrP$^C\to$PrP$^{Sc}$); MPN as a Poisson-based probability estimate from a positive/negative dilution pattern; capsid (± envelope) as the two fundamental virus architectures.

Why this works: Each sub-part probes a different mechanism by which a biological agent survives, is detected, or causes disease — filter-feeding bioaccumulation, self-propagating protein misfolding, culture-independent rapid detection, dilution-to-extinction statistics, and structural virology — that together span the breadth of food-microbiology hazard types.

Common pitfall: Listing culture-based plate counting as a "rapid method" (it is the traditional, slow reference method rapid methods are compared against); describing MPN as an exact count rather than a statistical estimate; drawing the envelope as part of the capsid itself rather than as a separate, host-derived outer membrane.

Source: Jay, Loessner & Golden, Modern Food Microbiology, seafood pathogens, prion, rapid-methods and MPN chapters.

Question 11 of 12: Shortanswer — Pathogens, Prions, Rapid Methods, MPN and Virus Structure

Section I — Food Chemistry