Bacteria

BSEN-625Advances in Food Engineering

Bacterial Evolution & Classification

Most numerous organisms on earth Earliest life forms Microscopic prokaryotes (no nucleus nor membrane-bound organelles) Contain ribosomesInfoldings of the cell membrane carry on photosynthesis & respiration Surrounded by protective cell wall containing peptidoglycan(protein-carbohydrate)

Bacterial Evolution & Classification

Many are surrounded by a sticky, protective coating of sugars called the capsule or glycocalyx (can attach to other bacteria or host) Have only one circular chromosomeHave small rings of DNA called plasmidsMay have short, hairlikeprojections called pili on cell wall to attach to host or another bacteria when transferring genetic material Most are unicellularFound in most habitats

Bacterial Evolution & Classification

Most bacteria grow best at a pH of 6.5 to 7.0 Main decomposers of dead organisms so recycle nutrientsSome bacteria breakdown chemical & oil spills Some cause diseaseMove by flagella, gliding over slime they secrete ( e.g. Myxobacteria)Some can form protective endospores around the DNA when conditions become unfavorable; may stay inactive several years & then re-activate when conditions favorable Classified by their structure, motility (ability to move),molecular composition, & reaction to stains (Gram stain)Grouped into 2 kingdoms --- Eubacteria (true bacteria) & Archaebacteria (ancient bacteria) Once grouped together in the kingdom Monera

Kingdom Eubacteria (true bacteria)Most bacteria in this kingdom come in 3 basic shapes --- cocci (spheres), bacilli (rod shaped), spirilla (corkscrew shape) Bacteria can occur in pairs ( diplo- bacilli or cocci) Bacteria occurring in chainsare called strepto- bacilli or cocciBacteria in grapelike clusters are called staphylococci Most are heterotrophic(can’t make their own food)

Shape of bacteria

Kingdom Eubacteria (true bacteria)Can be aerobic (require oxygen) or anaerobic(don’t need oxygen) Subdivided into 4 phyla ---Cyanobacteria (blue-green bacteria), Spirochetes, Gram-positive, & ProteobacteriaCan be identified by Gram staining (gram positive or gram negative)

Examples of bacteria

Gram Staining

Developed in 1884 by Danish microbiologist, Hans Gram Bacteria are stained purple with Crystal Violet & iodine; rinsed with alcohol to decolorize; then restained with Safranin (red dye)

Gram StainingBacterial cell walls either stain purple or reddish-pink

Gram-positive bacteriaThick layer of peptidoglycan (protein-sugar) complex in cell walls & single layer of lipids Stain purple Lactobacilli are used to make yogurt, buttermilk …. Actinomycetes make antibiotics like tetracycline & streptomycinDisease-causing gram + bacteria produce poisons called toxins

Gram-positive bacteriaClostridium causes tetanus or lockjawStreptococcus cause infections such as “strep” throat Staphylococci cause “staph” infections Also cause toxic shock, bacterial pneumonia, botulism (food poisoning), & scarlet fever Can be treated with penicillin (antibiotics) & sulfa drugs Streptococcus

Gram-positive bacteriaCell walls have a thin layer of peptidoglycan & an extra layer of lipids on the outside Stain pink or reddishLipid layer prevents the purple stain & antibiotics from entering (antibiotic resistant)Some are photosynthetic but make sulfur, not oxygenRhizobacteria grow in root nodules of legumes (soybeans, peanuts…) & fix nitrogen form the air for plants Rickettsiae are parasitic bacteria carried by ticks that cause Rocky Mountain spotted feverSpirochetes can cause syphilis & Lyme disease

Food Related MicrosGram-positive bacterium, motile by means of flagella. Some studies suggest that 1-10% of humans may be intestinal carriers of L. monocytogenes. It has been found in at least 37 mammalian species, both domestic and feral, as well as at least 17 species of birds and possibly some species of fish and shellfish. It can be isolated from soil, silage, and other environmental sources. L. monocytogenes is quite hardy and resists the deleterious effects of freezing, drying, and heat remarkably well for a bacterium that does not form spores. Most L. monocytogenes are pathogenic to some degree.

Listeria monocytogenes

Listeria monocytogenesHas been associated with such foods as:

raw milk, pasteurized fluid milk, cheeses (particularly soft-ripened varieties), ice cream, raw vegetables, fermented raw-meat sausages, raw and cooked poultry, raw meats (all types), and raw and smoked fish.

Its ability to grow at temperatures as low as 3°C permits multiplication in refrigerated foods. The 1987 incidence data prospectively collected by CDC suggests that there are at least 1600 cases of listeriosis with 415 deaths per year in the U.S. TThe vast majority of cases are sporadic, making epidemiological links to food very difficult

Food Related MicrosIs a rod-shaped, motile bacterium -- nonmotileexceptions S. gallinarum and S. pullorum--, nonsporeformingand Gram-negative. There is a widespread occurrence in animals, especially in poultry and swine. Environmental sources of the organism include water, soil, insects, factory surfaces, kitchen surfaces, animal feces, raw meats, raw poultry, and raw seafoods, to name only a few.

Salmonella typhimurium

Salmonella sspHas been associated with such foods as:

Raw meats, poultry, eggs, milk and dairy products, fish, shrimp,frog legs, yeast, coconut, sauces and salad dressing, cake mixes, cream-filled desserts and toppings, dried gelatin, peanut butter, cocoa, and chocolate. Various Salmonella species have long been isolated from the outside of egg shells. The present situation with S. enteritidis is complicated by the presence of the organism inside the egg, in the yolk. This and other information strongly suggest vertical transmission, i.e., deposition of the organism in the yolk by an infected layer hen prior to shell deposition. Foods other than eggs have also caused outbreaks of S. enteritidisdisease.

It is estimated that from 2 to 4 million cases of salmonellosisoccur in the U.S. annually.

Clostridium botulinumClostridium botulinum is an anaerobic, Gram-positive,spore-forming rod that produces a potent neurotoxin.The spores are heat-resistant and can survive in foods that are incorrectly or minimally processed. Seven types (A, B, C, D, E, F and G) of botulism are recognized, based on the antigenic specificity of the toxin produced by each strain. Types A, B, E and F cause human botulism. Types C and D cause most cases of botulism in animals. Animals most commonly affected are wild fowl and poultry, cattle, horses and some species of fish. Although type G has been isolated from soil in Argentina, no outbreaks involving it have been recognized.

Clostridium botulinumThe toxin is heat labile and can be destroyed if heated at 80°C for 10 minutes or longer. The incidence of the disease is low, but the disease is of considerable concern because of its high mortality rate if not treated immediately and properly. Most of the 10 to 30 outbreaks that are reported annually in the United States are associated with inadequately processed, home-canned foods, but occasionally commercially produced foods have been involved in outbreaks. Sausages, meat products, canned vegetables and seafood products have been the most frequent vehicles for human botulism. The organism and its spores are widely distributed in nature.They occur in both cultivated and forest soils, bottom sediments of streams, lakes, and coastal waters, and in the intestinal tracts of fish and mammals, and in the gills and viscera of crabs and other shellfish.