Conventional identification of Bacillus cereus sensu lato isolates from soil and milk powder samples

Published: 29-09-2019| Version 1 | DOI: 10.17632/832g5m5yvb.1
Rener De Jesus,
Gina Dedeles


A total of 12 Bacillus cereus sensu lato (s.l.) isolates were subjected to conventional biochemical tests, morphological, and colony identification. The isolates were obtained from soil and milk powder samples by the spread plate method using a highly selective Bacillus cereus agar (BCA) (Oxoid) supplemented with egg yolk emulsion and polymyxin B. The BCA plates were incubated at 30 degrees Celsius for 24 hr and observed the plates for typical colony growth. The B. cereus (presumptive) colonies on BCA appear turquoise-peacock blue colonies with precipitation. The bacterial cells under microscope are 4-5 micron long and 1.0 - 1.5 micron wide with square ends and rounded corners with spores located centrally or at paracentral position. The biochemical tests used here are based on the following observations: blood hemolysis, presence of hydrogen sulfide and indole, fermentation of lactose, ability to reduce nitrate, hydrolysis of starch, reaction with Voges-Proskauer medium, and motility (Table 1). Furthermore, cell morphology, gram reaction and presence of endospore were also conducted (Table 2). Checking for parasporal crystals and rhizoidal colonies were also included to identify B. thuringiensis or B. mycoides (Table 2). Additionally, the isolates were tested for the ability to grow at low temperature (6 degrees celsius) to identify psychotropic strains. To conclude, all isolates were identified as B. cereus sp. however since no genetic analysis was conducted, the 12 isolates were assigned as B. cereus s.l. None were identified as B. thuringiensis, B. mycoides or B. pseudomycoides.