Microorganisms: Bacteria

Bacteria are microorganisms. Microorganisms are very short single cell organisms which can observed only by microscope. They are invisible to the human eye and ubiquitous. Bacteria can be usefull. They help us in digesting, in fermenting food and in many other applications concerning food. Howewer, they are best known for being rather unhealthy: bacteria can be infectious and toxic.


Bacteria can be classified for example by shape (cocci, rods), by gram-stain (positive, negative), by the atmosphere they need (aerobic, anaerobic), and by their biochemical reactions.


_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Non-pathogenic bacteria

Special application: Food fermenting, probiotics, biomass, meat maturation. bioengineering

gram positive bacteria

1.) aerobic cocci

  • Micrococcus (Micrococcaceae): cocci; aerobic; used for ripening of ham and raw sausages;


2.) aerotolerant rods or cocci

  • Lactobacillaceae (milk-bacterium) Lactobacillus: rod; Lactobacillus acidophilus (acid loving milk-bacterium): used in yoghurt fermentation;
  • Streptococcus thermophilus: cocci in a chain; used in yoghurt fermentation;


gram negative bacteria

1.) aerobic rods 

  • Acetobacter: rod; aerobic; produces vinegar by alcohol fermentation;


Image: Lactobacillus on MRSM agar (source: D. Graubaum, Berlin, Germany)


_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Pathogenic bacteria

Pathogenic Bacteria are the source of many food-borne infections (see chapter Food Poisoning). The most common cause for infections is lacking hygiene.

gram positive bacteria

1.) facultative anaerobic cocci:

  • Staphylococcus auerus (Staphylococcaceae): cocci in grape-shape; living in the human respiratory tract and on the skin; produces toxins; symptoms are diarrhoea and vomitus without fever; sources can be raw eggs, pudding, raw milk;


2.) facultative anaerobic rods

  • Listeria monocytogenes (Listeriaceae): rods in small chains; sources are raw meat, raw milk, poultry; causes the infection of listeriosis; dangerous for pregnant women because it induces abortion;


3.) aerotolerant rods or cocci

  • Streptococcus mutans (Streptococcaceae): cocci in a chain; induces caries;


4.) endospore forming rods

  • Bacillus cereus (Bacillaceae): rod; aerobic or facultative anaerobic; endemic; soil-dwelling; sources are fried rice and cereals; causing nausea, vomiting, diarrhoea; produces bacterial toxin;
  • Clostridium botulinum (Clostridiacea): rod; obligate anaerobic; produces several toxins (best known: neurotoxin); induces botulism, a muscular paralysis; main source is tinned food;


gram negative bacteria

1.) aerobic rods

  • Pseudomonas  (Pseudomonadaceae): rod; found in soil, water and on skin; causes food spoilage; some species (P. aeruginosa) causing cross-infections in hospitals;


2.) microareophilic rods

  • Campylobacter (Campyolbacteriaceae): rod; mircoaerophilic; the main cause of food borne illnesses these days; symptoms are vomitus, diarrhoea, nausea; self limited;


3.) facultative anaerobic rods (Enterobacteriaceae)

  • Salmonella: rod; zoonosis; sources are raw meat, raw milk, raw eggs; induces enteritis illness;
  • Escherichia coli: rod; found in the intestine; some strains are pathogenic for humans, e. g. E. coli O157; some strains produces toxins (enterohaerogic E. coli, short EHEC, shigella like toxin building E. coli, short STEC); sources are similar to salmonella, but also sewage water


Image: Bacillus cereus on ST1-Agar (source: D. Graubaum, Berlin, Germany)

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _


This list of bacteria cannot be complete. It just gives a small overview on the world of food microbiology.


This text was prepared by Prof. Diana Graubaum, Department of Food Microbiology, Beuth University of Applied Sciences Berlin, Germany.

For further questions please refer to: graubaum@beuth-hochschule.de



                                   "Bacteria in Food"