S.pneumoniae - Biochemical Reactions, Resistance, Toxins and Virulence Factors

 Biochemical Reactions

• ·         Catalase and oxidase negative
• ·         Ferments sugars with acid production -Fermentation tested in Hiss’s serum water **
• ·         Ferments Inulin-streptococci does not  
• ·         Pneumococci are bile soluble-of diagnostic importance
• ·      Bile solubility test: If a few drops of 10% sodium deoxycholate are added to 1 mL of an overnight broth culture, the culture clears due to lysis of cocci- due to an autolytic amidase that cleaves the bond between alanine and muramic acid in the peptidoglycan; Amidase is activated by bile salts

          

**(Hiss's serum water is used in carrying out fermentation tests with certain pathogenic organisms, such as the pneumococcus that do not grow well in the peptone–water solutions. Fermentation is indicated by the change in the indicator by the production of acid that may also coagulate the serum. Litmus milk is used for biochemical tests. Acid production, a result of the fermentation of lactose, turns the medium pink and may also coagulate it; proteolytic activity is indicated by digestion and alkali formation.)

Resistance

• ·         Pneumococci are delicate organisms.
• ·         Readily destroyed by heat- 52° C for 15 minutes and antiseptics
•    Cultures die on prolonged incubation due to accumulation of toxic peroxides- maintained by lyophilisation
• ·         Sensitive to beta lactam antibiotics initially- now resistant-resistance due to alteration in penicillin binding proteins on the bacterial surface
• ·         Pneumococci sensitive to Optochin (Ethyl hydrocuprein) sensitive- Streptococci are not

Toxins and Virulence Factors

• ·         Virulence depends on its capsule and the production of a toxin called pneumolysin
• ·         Capsular Polysaccharide (acidic & hydrophilic) - protects the cocci from phagocytosis
• ·      Enhanced Virulence of type 3 due to abundant capsular material- Non capsulated strains are avirulent - Antibody to capsular polysaccharide is protective
• ·         Pneumolysin –membrane damaging toxin produced by Pneumococci- cytotoxic and complement activating properties- virulence factor-immunogenic
• (Pneumolysin-negative mutants showed less virulence in experimental animals)
• ·         Pneumococcal Autolysins- release bacterial components in infected tissues- contribute to virulence
•  Oxygen labile hemolysin and leucocidin – not significant in virulence