microbial detection methods

3 types of functions microbes have in food

production of food such as yogurt or wine
spoilage of raw/processed food
human illness

these are ....

Aerobic plate count (APC)

provides an estimation of number of aerobic microbes that can grow at 35±2˚C

Only includes number of aerobic molecules

Sample collection

methods varies based on regulatory agency, type of sample, bacteria to be tested

disadvantages of spread plate

coalescence of colonies (two or more colonies being close together)
Only 0.1 mL of sample can be plated on each plate

Colony forming Units (CFU)

In theory one bacterial colony on plate originated from one bacterial cell. This may not be always true, if homogenization was not performed correctly at every step

less agar media

what is the only advantage of using the roll tube method?

membrane filtration

liquid sample which has low bacterial population or contamination level. Large volume of sample can be filtered using a single 0.45 µm filter and bacteria from the sample can be concentrated on the filter surface

membrane filtration advantages

large volume of liquid samples can be concentrated
large volume of air samples can be concentrated
used for viscous samples such as milk and juice
used for water sample with low number of bacteria

Direct epifluorescent filter technique

process uses fluorescent dyes to stain bacterial cells which are then counted using fluorescence microscope. Then number of cells are back calculated by multiplying by dilution factor

Direct microscopic count (DMC)

This is the fastest method as no incubation is required; in the process the food/liquid sample is uniformly spread over 1-cm area on a microscopic slide

DMC advantages

little processing of samples
limited equipments

these are the ...

DMC disadvantages

detection of dead and live cells
cell clumps

these are the ....

surface testing

this involves tests like the swab test, direct contact agar method, sticky tape method, and sponge system

metabolism based assay

microorganism generate energy by metabolism/chemical reaction/oxidation-reduction reaction; the redox potential can be measured by indicator dyes

Food Safety Modernization Act

new standards for irrigation water and testing of irrigation water is required (USDA changed Salmonella testing guideline from 25g sample to 325g sample size)

true

t/f

enrichment time and enrichment media are selected based on pathogen being tested

immunologically methods

one of the most widely used rapid methods; antibody used can be either monoclonal (specific target) or polyclonal (cellular targets)

IMS advantages

concentrate target antigen/bacteria
remove impurities
can remove non-specific targets
can be combined with other sensitive methods

these are seen as ....

IMS Disadvantages

antibody can degrade in 1 year
when combined with highly sensitive methods such as NGS, it still binds to other antigens

these are seen as ...

latex agglutination assay

coating latex particle are coated with specific antibodies, upon contact with specific antigen produce visible clumps; method is very convenient and rapid but requires 10^6 CFU

Immunodiffusion

movement of protein through the agar and formation of an antibody-antigen complex that form a visible precipitation in a chamber