Cultural characteristics, biochemical properties and resistance

 

CULTURAL CHARACTERISTICS, BIOCHEMICAL PROPERTIES AND RESISTANCE

Cultural characteristics

  • They are aerobic and facultative anaerobic, growing readily on simple media over a pH range of 6.8. Optimum temperature for growth is 370C.
  • On nutrient agar the colonies are large, circular, low convex and smooth.
  • The selective enriched media for salmonellae are tetrathionate broth, selenite broth and rappaport vasiliadis medium.
  • The host adapted serotypes from pigs and poultry are more fastidious than others.
  • They do not tolerate selenite broth and tetrathionate broth. In this case, Rappaport is highly suitable.
  • After 24-48 hr incubation on selective broth the subculture will be made on MacConkey agar, Brilliant green agar, XLD and Salmonella Shigella (SS) agar.
  • The majority of salmonellae, except some strains of S.arizonae, are non-lactose fermenters and produce pale or colorless colonies on MacConkey agar.
  • Most salmonellae give an alkaline reaction in brilliant green agar and have red colonies.

Growth of Salmonella on EMB agar

  • On XLD medium, they produce H2S and have red colonies with a black center (Black center with red skirt).
  • On Salmonella and Shigella agar they produce colorless colonies with black center.
  • The typical reaction for salmonellae in TSI (triple sugar iron) agar is an alkaline slant (red), acid butt (yellow) and superimposed H2S (black) production (R/Y/H2S+). 0610_VMC311_Triple_sugar_iron_agar_test.swf
  • The test for lysine decarboxylation is positive.

Growth of Salmonella on TSI agar

Biochemical properties

  • Salmonella gives IMViC test -, +,-,+. They ferment maltose, mannitol, mannose and glucose and produce acid and gas. But do not ferment lactose, sucrose and salicin.
  • Urease negative. Most salmonellae produce H2S except S.cholera suis and S.paratyphi A.
  • Salmonella pullorum ferments glucose and rhamnose while S.gallinarum ferments dulcitiol and maltose.

Resistance

  • The bacilli are destroyed at 55 oC in one hour or at 60 oC in 15mts.
  • Boiling or chlorination of water and pasteurization of milk destroys the bacilli.
  • Cultures may be viable for years if prevented from drying.
  • They are killed within 5 minutes by mercuric chloride (1:500) or 5% phenol.

Last modified: Monday, 4 June 2012, 5:17 AM