1.     Gram positive genera:


-       Bacillus; aerobic sporeforming rods, meso-, psychro- and thermophilic species exist.  This genus includes important spoilage species as well as pathogenic species which cause anthrax (B. anthracis) and B. cereus which can cause foodborne gastroenteritis.  Due to the resistance of endospores to drying and most other stresses, aerobic sporeformers are widely distributed in nature but it is generally accepted that the primary habitat of most Bacillus species is soil from which they contaminate everything by distribution in dust.


-       Clostridium; anaerobic sporeforming rods.  Like Bacillus, meso-, psychro- and thermophilic species exist.  The most notorious species are C. botulinum which causes botulism, and C. perfringens, a common cause of gastroenteritis.  Clostridia are ubiquitous, they are found in soil, sewage, decaying vegetation, animal and plant products, and in the intestinal tract of man and animals.


-       Lactobacillus; very diverse genus of fastidious acid tolerant (some can grow at pH<3.0), microaerophilic, nonsporeforming rods.  Mesophilic but some species can grow below 4oC or at temperatures over 50oC.  Lactobacilli are found in low numbers on all plant surfaces and are also an important part of the natural intestinal microbiota.  They can be isolated from raw milk or the surface of uncooked meats.  Lactobacilli are included among the lactic acid bacteria and this genera includes species used to ferment dairy, vegetable and meat products as well as other species that are responsible for spoilage of acid foods and meats.


-       Lactococcus; another genus that is included in the lactic acid bacteria.  These are mesophilic (grow at 10oC but not at 45oC), acid tolerant cocci found on plant surfaces and in raw milk.  They include L. lactis which is an important species in dairy fermentations.


-       Leuconostoc; another genus in the lactic acid group.  These mesophilic cocci include species important to dairy, vegetable, and wine fermentations.


-       Listeria; nonsporing rods, aerobic or facultatively aerobic.  Mesophilic but some species and strains can grow at temperatures as low as 1oC or as high as 45oC.  Listeria spp. are widely distributed in nature and can be isolated from water, mud, vegetation and from human and animal waste.  All species of Listeria are killed by heating at 60o for 30 min.


This genus includes the important pathogen L. monocytogenes that causes listeriosis in humans.  Most people are immune but the mortality rate among susceptible persons can be high: in 1980Ős 34% in UK, 28% in U.S.  Concern over this organism is very real.  In June, 1996, over 28,000 pounds of Swiss-type Jarlsburg cheese made in Washington were recalled from stores in 9 states because L. monocytogenes had been found in some samples.


-       Staphylococcus; Mesophilic, facultatively anaerobic cocci.  Most strains can grow in 10% NaCL and between 18 and 40oC.  Staphylococci are primarily associated with skin glands, and mucus membranes of warm-blooded animals.  S. aureus and some other species produces a heat stable enterotoxin (some toxins survive 16 h @ 60oC) that is a major cause of food poisoning, usually as a result of poor sanitation or hygiene.


  1. Gram-negative genera:


-       Campylobacter, Microaerophilic to anaerobic spirally curved rods.  Do not ferment or oxidize carbohydrates, energy is obtained from amino acids or TCA cycle intermediates.  Found in the intestinal and reproductive tracts of man and animals.  This genus contains several pathogenic species such as C. jejuni, which in 1991 caused nearly as many cases of gastroenteritis in the U.S., but more total deaths from those cases, than the more notorious Salmonella.


-       Escherichia; Most thoroughly studied genus of bacteria.  Facultatively anaerobic rods that are found in the lower intestine of warm-blooded animals.  Because E coli are associated with fecal contamination, this species is used as an indicator of food safety and sanitation.


Some strains of E. coli cause food poisoning and are the leading cause of travelers diarrhea.  A recent pathogen to emerge is E coli O157:H7, and this organism created headlines at least twice in 1996; in Japan, over 10,000 cases were tied to their school lunch program and a recent U.S. outbreak was associated with unpasteurized apple juice products.  At least 20 cases of O157:H7 infection were documented in Utah in 1996.


-       Hafnia; Mesophilic, facultatively anaerobic rods found in human and animals waste, and in soil or water.  Only one species, H. alvei, and it is important because of its ability to spoil refrigerated meat and vegetable products.


-       Pseudomonas; Without question the single most important food spoilage group of bacteria Aerobic rods, meso- and psychrophilic species, found in soils and water.  Most it not all cannot grow at pH<4.5.  Few Nutritional requirements – some can even use H2 or CO as energy sources!  Several species produce pigmented compounds that fluoresce.


-       Salmonella; Mesophilic, facultatively anaerobic rods.  All members of this genus are considered pathogenic, causing fever, gastroenteritis and septicemia.  Salmonella have not been separated into different species but instead are grouped into subgenera which contain related serovars defined by antigenic properties.  This classification scheme is complicated by the ability of some bacteriophages to convert one serovar to another (phage conversion).  Salmonella are intestinal organisms transmitted to humans by fecal contamination of food and water.


-       Shigella; Like Salmonella, members of this genus are mesophilic, facultatively anaerobic rods and all are considered pathogenic.  These bacteria are intracellular parasites of the epithelium of humans and higher primates.


-       Vibrio; Facultatively anaerobic, mesophilic (although some can grow at 4oC) rods found in fresh and salt water.  Growth of all species is stimulated by Na+ ions.  V. cholera is the causative agent for epidemic cholera and several other species are serious pathogens.  Consumption of raw shellfish and fish are often linked to vibrio outbreaks.


-       Yersinia; Facultatively anaerobic, mesophilic rods that can grow anywhere between 4-42oC.  Psychrophilic growth has also been recorded in some strains.  This genus includes Y. pestis, the cause of plague, and Y. enterocolitica which causes gastroenteritis.  Yersinia spp. are widely distributed in soil and water and are also found in the intestinal tracts of animals.