FOOD 3005 Study Guide - Final Guide: Food Microbiology, Microorganism, Aureus
2 May 2018
School
Department
Course
Professor
Food microbiology
Intro
➢ Fundamental biological discoveries still influence Food Microbiology today:
- Use of agar-based media to culture microorganisms
- Use of microscopes to see microbes
- Kohs postulates to poe ipliatio of ioogaiss i pheoea relating to food
microbiology
➢ Molecular biology and other new methods have begun to impact our understanding of food
microorganisms
➢ Food microbiology is a young field of study relative to other scientific disciplines
Four basic detection and enumeration methods:
1. Plate count (total plate counts)
- identity of cell not known
- quantify cell population; greater than 250 cell/ml in liquids and 2500 cells/g in solids
- assumes every cell; forms one colony, every colony originates from one cell + see with naked
eye
-‘ealit: soe ot go o atifiial edia; olo iitiated fo a ells = olo foig
unit (CFU) and physical factors affect the cell producing macroscopic
- most used technique, only count viable cells and ones that grow in test conditions
- basic technique
homogenize a food sample in buffer to give 10-fold dilution
do a series of to-fold dilutions and spread onto agar plates
incubate
count the average number of colonies per plate (only use ones containing 25-250)
multiply by dilution factor to give the number of cells in original sample
2. Selective media or Differential media
- Selective: ingredients in medium may favour growth of targeted microorganism and/or inhibits
the growth of competing microorganism
medium containing antibiotics, would inhibit the growth of bacteria and be selective
for non-bacterial microorganism (yeast and molds)
- Differential: design to make target microorganism look different or standout from background
Staphylococcus aureus colonies are black and have a precipitation zone on Baird-
Parker agar
- positives; help with identification and enumeration of specific microbes
- negatives; identity must be confirmed with biochemical or genetic methods and other
microorganisms can grow on selective media
3. Most probable Number
- estimate low numbers of microorganism (less than 50)
- want to enumerate them to make sure they are below acceptable limits
- basic technique for 3-tube
1 mL of sample to three test tubes containing selective media
do series of three 10-fold dilutions
find more resources at oneclass.com
find more resources at oneclass.com
incubate, record growth; (-) = no turbidity or (+) = turbid
use pattern of turbidity with statistical MPN tables
4. Enrichment techniques
- zeo toleae
- dot eed to out the
-must find one of these cells hiding among millions of other microbial cells
- basic technique
use preenrichment media: non-selective medium that allows injured target cells to
repair, recover and regain resistance to selective agents
selective enrichment: promote high growth levels of the target organism while
inhibiting growth of other microorganisms
isolate target pathogen on selective agar
confirm identity by biochemical or molecular methods
Injury
**Sublethal treatment = microbial cells may be injured but not killed
- do not grow on media
less resistant to selective agents and have higher nutritional requirements
- can recover with time or when ingested and pathogens can still cause disease
- preenrichment media needed to detect the presence of pathogens
Viable but nonculterable (VNC) cells
- cannot be cultured by normal methods
- other methods; microscopic methods, specific fluorescent probes or dyes, biochemical or genetic
markers
Intrinsic factors
- pH
many microbial cells have a pH near neutrality
in acidic environment, the microorganism must expend energy to keep its internal pH from
becoming acidic
spends energy controlling internal pH, has less energy to grow, cause infection/produce toxin
** Low pH food requires less processing
most food illness pathogens stop growing or die below pH 4.4
lactic acid bacteria and molds stop growing or die below pH 3.0
most microorganisms stop growing or die above pH 8-9
- aw
availability of water in food determines microbial growth; water bound to food compounds
and hence, not available to microbes
some food have the same aw but different moisture content
high conc of food components that bind water in dried fruit allow a low aw with high moisture
salt and sugar lower aw
lowest aw that supports growth of food illness pathogens = 0.85
- nutrients
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find more resources at oneclass.com
Extrinsic factors
- temperature
keep microorganisms outside of their normal temperature range
4-60 rule; below 4 °C or heated above 60 °C and the time spent at a particular temp
Hurdle Technology
- combining various inhibition or killing factors to limit or eliminate harmful microorganisms
- this involves less salt and chemicals in food
- high level single hurdle
forceful treatments can also damage the food
-lower levels of multiple hurdles (Hurdle Technology)
non-lethal levels of various factors can be used in combination to stop growth without
damaging the food
* The level of acidity (hurdle 1) and water activity (hurdle 2) in food determines the storage temperature
(hurdle 3)
Bacterial Spores
- dormant, tough and non-reproductive structure
- ensure survival of a bacterium through periods of environmental stress
- resistance to multitude of stresses
Radiation, desiccation, some enzymes, temperature, nutrient deficit and chemicals
- most are produced inside a vegetative mother cell (endospores) when the environmental conditions
become adverse
Bacterial Spore Formation
1. Vegetative cell
2. Asymmetric division
3. Engulfment of forespore in mother cell
4. Cortex and coat synthesis
5. Spore maturation and mother cell lysis
Hurdle not cleared = no growth
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Fundamental biological discoveries still influence food microbiology today: Use of agar-based media to culture microorganisms. Ko(cid:272)h(cid:859)s postulates to p(cid:396)o(cid:448)e i(cid:373)pli(cid:272)atio(cid:374) of (cid:373)i(cid:272)(cid:396)oo(cid:396)ga(cid:374)is(cid:373)s i(cid:374) phe(cid:374)o(cid:373)e(cid:374)a relating to food microbiology. Molecular biology and other new methods have begun to impact our understanding of food microorganisms. Food microbiology is a young field of study relative to other scientific disciplines. Four basic detection and enumeration methods: plate count (total plate counts) Quantify cell population; greater than 250 cell/ml in liquids and 2500 cells/g in solids. Assumes every cell; forms one colony, every colony originates from one cell + see with naked eye. Ealit(cid:455): so(cid:373)e (cid:449)o(cid:374)(cid:859)t g(cid:396)o(cid:449) o(cid:374) a(cid:396)tifi(cid:272)ial (cid:373)edia; (cid:272)olo(cid:374)(cid:455) i(cid:374)itiated f(cid:396)o(cid:373) (cid:373)a(cid:374)(cid:455) (cid:272)ells = (cid:272)olo(cid:374)(cid:455) fo(cid:396)(cid:373)i(cid:374)g unit (cfu) and physical factors affect the cell producing macroscopic. Most used technique, only count viable cells and ones that grow in test conditions. Homogenize a food sample in buffer to give 10-fold dilution. Do a series of to-fold dilutions and spread onto agar plates.
