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Indicators in Food Microbiology

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Unit 2 summary- Indicators in Food Microbiology


  • Purpose: safety & shelflife
  • Looking for a pathogen is like looking for a needle ūü°™¬†look for indicators instead (if indicators are present, spoilage are present)
  • Provide a gauge of product shelf life
  • Highlight 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 interest
  • Shares the same niche as the pathogen
  • Can be enumerated inexpensively and w. simple techniques
  • Can be differentiated against background microflora (ease of detection)
  • Non-pathogenic

Attributes for Quality indicators

+Negatively relate to food quality ¬†(when count is highūü°™¬†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 )

  1. Spoilage (quality) indicators [pic 1]


Total Aerobic Count

Psychrotrophic Count ūü°™¬†pseudomonas

Lactic acid bacteria

Yeast and molds

Types of counts will depend on the nature of the product

1) 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 ūü°™¬†dangerous for raw milk (lipid degradation produce ketone ūü°™¬†off-flavor)

2) Psychrotrophic Count 

- can grow at low temp, spoilage of refrigerated food, estimate shelf life

  • Listeria & Pseudomonas; also incl. yeast and molds
  • Important b/c psychrophs are easier to cultivate, foods are commonly refrigerated

3) Pseudomonas

  • Potent spoilers, very versatile (grow on anything)
  • Obligate aerobe ūü°™¬†vacuum packaging/modified atm mainly to stop pseudomonas growth
  • Grow in low temp & forms polysacc ūü°™¬†form biofilm¬†(difficult to remove and easy to contaminate)

Enzymes produced by pseudomonas

  1. Proteinases- break down proteins leading to generation of ammonia, sulfur and/or organic acids (butyric, acetic).
  2. lipolytic - hydrolyze triglycerides and accelerate lipid oxidation (rancidity).
  3. 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 ‚Äď Flourescent Pseudomonas release siderophores¬†(pigmented/fluor‚Äô) to assimilate iron

*Meat & eggs have low Fe and enhance siderophore production in Pseudomonas.

   Important features of pseudomonas: quorum sensing & biofilm formation

cell-cell communication to coordinate activities

gram neg & pos (cyclic peptides)

cell density affect

why quorum sensing important? (*enable bac population to act collectivelyūü°™¬†more efficient use of cell resourcesūü°™¬†can overcome defenses more effectively)

  • Increase spoilage potential of populations.
  • Stimulate biofilm formation.
  • Increase virulence of pathogens


  • Opportunistic pathogens
  • P. aueruginosa ¬† --lung disease
  • Spoilage (typically not pathogenic)
  • P. fluorescens ¬†--fresh produce
  • P. putrefacines -- meat[pic 3]
  • P. fragi

4) Lactic acid bacteria- Gram positive non-spore forming rods / cocci

Genera 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 acidūü°™¬†phenotypic¬†classification, not genetic)
  • Widespread(plants & GI of animals) but difficult to culture
  • Fastidious¬†(complex nutritional demand)
  • Tolerate low pH and high ethanol
  • Since they tolerate many stress, refrigeration is the key (<10C) (sorbic/benzoic acid can‚Äôt eliminate them)
  • High lactic acid bac indicate temperature abuse problem
  • Can be beneficial or spoiling depending on fermentation products & food type


  1. Homofermentative (ferment carbohydrates to predominantly lactate)
  • Lactobacillus planetarium¬†& Lactobacillus delbrueckii
  1. Facultative homofermenters (used extensively in dairy fermentation) (prefer homolactic fermentation but can perform heterolactic)
  • Lactococcus lactis subsp lactis
  1. Heterofermentative (real problems!) (ferment carbohydrates to>1 products i.e. lactate, acetate and ethanol, CO2)          Kefir sourness: acetate
  • Leuconostoc paramesenteroides¬†& Lactobacillus brevis

[pic 4][pic 5][pic 6]

Probiotics ‚Äď live microbes that when administered in sufficient amt confer a health benefit to the host.


  • Safe
  • Viable¬†* (10^9 CFU/serving)
  • Provide benefits (vague definition)
  • Bile resistant (survive bile salt in GI tractūü°™¬†viable)
  • Acid resistant

 Microbes used for human consumption



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