Capsule

  1. Gelatinous layer surrounding the bacterial cell; gives many bacterial colonies a mucoid appearance.
  2. Composed of polysaccharide (except in Bacillus anthracis) which varies among bacteria and is used in identification.
  3. 4 reasons the capsule is important:
    • It is a virulence factor; inhibits phagocytosis.
    • Antibodies vs. capsular polysaccharides used to identify bacteria.
      • Quellung reaction used in clinical labs—when homologous antibody present, capsule swells.
    • Capsular polysaccharides elicit protective antibodies as antigens in certain vaccines; Ex.  Capsular polysaccharides from 23 types of Streptococcus pneumoniae are in the current vaccine.
    • It may play a role in bacterial adherence to human tissues and in formation of biofilms.

Flagella

  1. Long, filamentous appendages that propel bacteria towards nutrients and other attractants (chemotaxis).
  2. Composed of multiple copies of a single protein, flagellin organized in intertwined chains.
  3. Proton motive force provided by ATP provides energy for movement.
  4. Present in characteristic numbers (1 or many) and arrangements (at the end or all over the bacterial surface).
  5. Most common in rods, not cocci.
  6. May play a role in pathogenesis; Ex. coli and Proteus vulgaris are common causes of urinary tract infections (UTIs), perhaps because their flagella move them up the urethra into the bladder.
  7. Antibodies vs. flagellar proteins are used to identify some bacterial species; Ex. Salmonella

 

Pili (Fimbriae)

  1. Hair-like filaments extending from cell surface.
  2. Shorter, straighter than flagella.
  3. Composed of the protein, pilin, organized in helical strands.
  4. Found mainly on Gram-negative organisms.
  5. Mediate attachment of bacteria to specific receptors on human cells; can be considered a virulence factor.
  6. Sex pili function during conjugation.

 

Glycocalyx (slime layer)

  1. Polysaccharide coating secreted by many bacteria that mediates adherence to skin, heart valves, catheters, etc. even teeth (basis of plaque and cavity formation); i.e. formation of biofilms.  Similar to a  capsule, but more loosely constructed.

Spores

  1. Highly resistant structures formed intracellularly by certain bacteria (Gram-positive bacilli: Bacillus and Clostridium) when nutrients are scarce.
  2. Composed of bacterial DNA, small amount of cytoplasm, cell membrane, peptidoglycan, a little water, and a thick, keratin-like coat.
  3. The outer coat provides remarkable resistance to heat, dehydration, radiation and chemicals; likely due to dipicolonic acid, a calcium ion chelator found only in spores.
  4. Have no metabolic activity and can remain dormant for years.
  5. When water and nutrients become available, spore may germinate and give rise to a single bacterial cell.
  6. Important features of spores and their medical implications
Important Features of Medical Implications
Highly resistant to heating:  are not killed by boiling (1000C), but are killed at 1210C. Medical supplies must be heated to 1210C for at least 15 minutes to be sterilized.
Highly resistant to many chemicals, including most disinfectants. Must use “sporicidal” solutions to kill spores.
Can survive for many years, especially in soil. Wounds contaminated with soil can get infected with spores and cause diseases such as tetanus and gas gangrene (caused by Clostridium species).
Metabolically inactive. Antibiotics are ineffective against spores; plus they cannot penetrate coat.
Formed when nutrients are limited, but germinate when nutrients become available. Usually see bacteria, not spores, in specimens from wounds (why?).
Produced by members of only 2 medically important genera, Bacillus and Clostridium (Gram-positive rods). Infections resulting from spores can be attributed to either Bacillus or Clostridium species.

 

 

Photo by adonofrio (Biology101.org)

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