Indicators in Food Microbiology
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Unit 2 summary- Indicators in Food MicrobiologyIndicatorsPurpose: safety & shelflifeLooking for a pathogen is like looking for a needle 🡪 look for indicators instead (if indicators are present, spoilage are present)Provide a gauge of product shelf lifeHighlight potential hazards An assessment of the previous Hx of food product (temperature abuse/ contamination) Evaluation of the efficacy of control measures to prevent and/or inactivate microbial activity (pasterization)*Expiry date (for nutra/pharmaceutical) vs. best before date (min time that ensure quality)Attributes to indicator summary: Always present when the pathogen is present Higher resistance/persistence than the pathogen of interestShares the same niche as the pathogenCan be enumerated inexpensively and w. simple techniques Can be differentiated against background microflora (ease of detection)Non-pathogenicAttributes for Quality indicators+Negatively relate to food quality (when count is high🡪 low quality)+Present in spoiled food Attributes for safety indicators+Present when pathogen is present & ideally, absent when pathogen is absent (avoid false pos & false neg )Spoilage (quality) indicators [pic 1] Include: Total Aerobic CountPsychrotrophic Count 🡪 pseudomonas Lactic acid bacteriaYeast and moldsTypes of counts will depend on the nature of the product1) Total Aerobic Count (standard plate count)- General assessment of bac; high #–> significant bac activity, conditions plates are incubated under temp that reflect food envir[pic 2] >5 log cfu/g is very high 6-7 ordours but not visible 9-10 visible Even Low count 🡪 dangerous for raw milk (lipid degradation produce ketone 🡪 off-flavor)2) Psychrotrophic Count – can grow at low temp, spoilage of refrigerated food, estimate shelf lifeListeria & Pseudomonas; also incl. yeast and moldsImportant b/c psychrophs are easier to cultivate, foods are commonly refrigerated3) PseudomonasPotent spoilers, very versatile (grow on anything)Obligate aerobe 🡪 vacuum packaging/modified atm mainly to stop pseudomonas growthGrow in low temp & forms polysacc 🡪 form biofilm (difficult to remove and easy to contaminate)Enzymes produced by pseudomonasProteinases- break down proteins leading to generation of ammonia, sulfur and/or organic acids (butyric, acetic).lipolytic – hydrolyze triglycerides and accelerate lipid oxidation (rancidity).Pectolytic- breakdown plant cell walls leading to loss of turgor pressure.*Proteases, lipases and pectinases associated with other bacteria and molds but Pseudomonas commonly implicated. Pigmentation – Flourescent Pseudomonas release siderophores (pigmented/fluor’) to assimilate iron*Meat & eggs have low Fe and enhance siderophore production in Pseudomonas. Important features of pseudomonas: quorum sensing & biofilm formationcell-cell communication to coordinate activities
gram neg & pos (cyclic peptides)cell density affect why quorum sensing important? (*enable bac population to act collectively🡪 more efficient use of cell resources🡪 can overcome defenses more effectively)Increase spoilage potential of populations.Stimulate biofilm formation.Increase virulence of pathogensRepresentativesOpportunistic pathogensP. aueruginosa –lung diseaseSpoilage (typically not pathogenic)P. fluorescens –fresh produceP. putrefacines — meat[pic 3]P. fragi4) Lactic acid bacteria- Gram positive non-spore forming rods / cocciGenera of significance Lactococcus Carnobactrium (resistant to irradiation)Lactobacillus Leuconostoc Bifidobacteria (probiotic, sensitive to O2 and stress)Pediococcus (probiotic)Streptococcus Why difficult to control?Facultative anaerobes, very diverse (common: produce lactic acid🡪 phenotypic classification, not genetic)Widespread(plants & GI of animals) but difficult to cultureFastidious (complex nutritional demand)Tolerate low pH and high ethanolSince they tolerate many stress, refrigeration is the key (<10C) (sorbic/benzoic acid can’t eliminate them)High lactic acid bac indicate temperature abuse problemCan be beneficial or spoiling depending on fermentation products & food type Metabolism:Homofermentative (ferment carbohydrates to predominantly lactate) Lactobacillus planetarium & Lactobacillus delbrueckiiFacultative homofermenters (used extensively in dairy fermentation) (prefer homolactic fermentation but can perform heterolactic)Lactococcus lactis subsp lactisHeterofermentative (real problems!) (ferment carbohydrates to>1 products i.e. lactate, acetate and ethanol, CO2) Kefir sourness: acetateLeuconostoc paramesenteroides & Lactobacillus brevis[pic 4][pic 5][pic 6]Probiotics – live microbes that when administered in sufficient amt confer a health benefit to the host.CharacteristicsSafeViable * (10^9 CFU/serving)Provide benefits (vague definition) Bile resistant (survive bile salt in GI tract🡪 viable)Acid resistant Microbes used for human consumption