A colony is a cohesive mass composed of many millions of bacterial cells, growing on or in a medium, such as blood agar, as a result of the multiplication and division of a single cell. The size, color, shape, edge, topography, consistency, and odor of the colony vary with each organism.
Three special structures assist in the classification of bacteria. The capsule is a gummy, gelatinous, or mucoid structure surrounding certain bacteria. The spore is an inactive, resting, and resistant form produced within the organism, usually as a result of unfavorable environmental conditions. The third and final special structure is the flagellum, which is a hairlike structure that provides motility.
Toxins generally are waste products of metabolism in a bacterial cell. Some bacteria produce toxins that attack red blood cells in a medium such as blood agar.
Grams stain reactions can either be positive or negative. Gram-positive reactions will stain dark blue-black. Gram-negative reactions will stain deep pink or reddish.
Bacteria are named by genus and species. The first word (capitalized) indicates the genus; the second word (not capitalized) indicates the species, which is a subdivision of the genus.
EXAMPLE: GENUS SPECIES
Gram-positive cocci (stain dark blue with Grams stain)
1. Streptococcus pneumoniaecauses pneumonia
2. Streptococcus pyogenes (Beta Streptococci Group A)causes strep throat
3. Staphylococcus aureumost common cause of boils and furuncles as well as osteomyelitis, pneumonia, septicemia, endocarditis, and impetigo
Gram-negative cocci (stain dark pink with Grams stain)
1. Neisseria gonorrhoeae (gonococcus) causes gonorrhea
2. Neisseria meningitidis (meningococcus) causes meningitis
1. Corynebacterium diphtheriaecauses diphtheria
2. Ciostridium (all are anaerobic and spore formers)
a. C. perfringens (welchii)causes gas gangrene
b. C. septicumcauses gas gangrene
c. C. tetanicauses tetanus or lockjaw
d. C. botulinumcauses food poisoning (botulism)
1. Yersinia (Pasteurella) pestiscauses bubonic or pneumonic plague
2. Brucella abortuscauses undulant fever (brucellosis)
3. Bordetella pertussiscauses whooping cough
Intestinal (Enteric) Gram-Negative Bacilli
1. S. typhicauses typhoid fever
2. S. paratyphi A & Bcauses paratyphoid fever
3. S. newport (S. enteritidis)causes gastroenteritis
Shigellaall of these cause bacillary dysentery (shigellosis).
1. S. dysenteriae (group A)
2. S. flexneri (group B)
3. S. boydii (group C)
4. S. sonnei (group D)