Food Microbiology

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Introduction 2(with e.g. nitrate as electron acceptor) or fermentation. Oxygen is often toxicfor anaerobic cells. Facultative anaerobic cells use oxygen and aerobicmetabolism if oxygen is available but switch to anaerobic metabolism inabsence of oxygen. Microaerophilic cells require low concentrations ofoxygen, while normal air contact is inhibitory. Lactic acid bacteria (e.g.Lactobacillus and Lactococcus) have an obligately anaerobic metabolism butare still resistant to oxygen.Table 2.1. Some of the bacterial groups (according to Bergey’s Manual of DeterminativeBacteriology) which are commonly encountered in food microbiology.Group DescriptionFood related organismsnr2 Gram-neg., aerobic, mobile, vibrioformedCampylobacter4 Gran-neg., aerobic rods or cocci Pseudomonas, Shewanella, Legionella5 Gram-neg., facultatively anaerobicrodsFamily Enterobacteriacae(e.g. Escherichia, Enterobacter,Salmonella, Shigella, Yersinia, Erwinia)Vibrio17 Gram-pos. cocci Staphylococcus, Streptococcus,Lactococcus, Enterococcus, Micrococcus,Leuconostoc18 Gram-pos endospore formersaerobic or facultatively anaerobic: Bacillusobligate anaerobes:Clostridium19 Gram-pos, non-sporulating rods LactobacillusBrochothrixListeriaThere is a number of often used group names of microorganisms. Some foodrelated examples are:”Gram-negative psychrotrophic rods”: This includes the genera Pseudomonas,Achromobacter, Alcaligenes, Acinetobacter, and Flavobacterium.”Lactic acid bacteria” (LAB) includes the food related genera Lactobacillus,Lactococcus, Pediococcus och Leuconostoc.”Coliform bacteria” is not synonymous to E. coli but includes Escherichia coliand Enterobacter.S.-O. Enfors: Food microbiology
Introduction 3A special problem with the microbial taxonomy is that the names often are “datedependent” due to repeated re-classification of species. One example is thelactic acid bacteria which previously were called Streptococcus lactis,Streptococcus cremoris a.o. These so called “lactic streptococci” are nowreferred to a new genus and galled Lactococcus lactis, Lactococcus cremorisetc. Other previous Streptococcus spp. wich are associated with the intestinesare now called Enterococcus, while yet another group of the previousStreptococcus genus remain as Streptococcus. When it comes to pathogenicorganisms a further classification problem is that only some strains of a certainspecies may be pathogenic while other strains are harmless. An example isEscherichia coli to which species the feared EHEC (enterohaemorrhagic E. coli)belong. In such cases immunological or DNA analyses are required for properclassification.Streptococcus is a genus with species of very different impact for humans. Someof todays Lactococcus and Enterococcus were previously classified asStreptococcus. They were then referred to as the lactic group and the entericgroup of the streptococci, respectively. A classification of the old streptococciaccording to current nomenclature is:1. Lactococci (Lactococcus lacits, L. cremoris a.o.). These organisms are oftenused for fermentation of food.2. Enterococci (Enterococcus faecalis, E. faecium a.o.) are in most cases notpathogenic, but certain strains have been reported to cause serious infections.Such contradictions are due to the limitation in the current nomenclature which isbased on phenotypic properties. These organisms are common in the intestinalflora. The presence of enterococci in food is not considered to be a health risk perse, but it is used as an indication of bad hygiene and that constitutes a risk, sinceother organisms of faecal origin like Salmonella may be present. For this reasonenterococci (together with the coliforms) are called indicator bacteria.3. Hemolytic streptococci. There are two types of hemolytic streptococci, andthese organisms remain in the genus Streptococcus: α-hemolytic and ß-hemolytic.The α-hemolytic streptococci are named the viridans group and they are commonon mucous membranes in the mouth and respiratory tract and on the teeth. The ß-hemolytic streptococci are named the pyogenes group and among them there areserious pathogens involved in several diseases and wound infections. α-hemolyticorganisms produce a greenish discolorisation zone around the colonies on bloodagar while ß-hemolytic cells produce a clear zone.Lactic acid fermentation is to a large extent also employed for production offood, namely some of the fermented foods: cheese, yoghurt, fermentedsausages, and fermented vegetables like sauerkraut, pickles, olives, and others.S.-O. Enfors: Food microbiology
Page 1 and 2: FoodMicrobiologySven-Olof EnforsKTH
Page 3: 1Chap 1IntroductionLiving organisms
Page 8 and 9: 2. The ecological basis of food spo
Page 24 and 25: 22Chapter 3. Spoilage of different
Page 26 and 27: 3. Spoilage of different types of f
Page 40 and 41: 38Chapter 4.Foodborne pathogensMost
Page 42 and 43: 4. Foodborne pathogens 40S. aureus
Page 44 and 45: 4. Foodborne pathogens 42Mycotoxins
Page 46 and 47: 4. Foodborne pathogens 44proved can
Page 48 and 49: 4. Foodborne pathogens 46typhi and
Page 50 and 51: 4. Foodborne pathogens 48EPEC are s
Page 52 and 53: 4. Foodborne pathogens 50through di
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5. Food preservation 52agents. It t
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5. Food preservation 54longer time.
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5. Food preservation 56Fig 5.4. Hea
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5. Food preservation 58Table 5.1 Ex
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5. Food preservation 60the exponent
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5. Food preservation 62Fig 5.9 Inac
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5. Food preservation 64In analogy w
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5. Food preservation 66Table 5.5 Fo
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5. Food preservation 68reduced intr
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5. Food preservation 70The exact me
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5. Food preservation 72temperature
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6. Fermented foods 74Table 6.1 Ferm
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6. Fermented foods 76hydrolyse the
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6. Fermented foods 78Table 6.3 Temp
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6. Fermented foods 80oxidations in
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6. Fermented foods 826.2.1 Fermente
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6. Fermented foods 846.2.2 Cheese.
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6. Fermented foods 86are also added
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6. Fermented foods 88degradation pr
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Food Microbiology

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