EVALUATION OF THE SEMISOLID POSTGATES-B MEDIUM FOR ENUMERATING SULFATE-REDUCING BACTERIA

Traditionally, sulfate-reducing bacteria (SRB) in environmental samples are enumerated by serial dilution in completely anaerobic liquid media which require time consuming anaerobic techniques for media preparation, inoculation and incubation. Moreover, an incubation time of up to 28 days is recommended for SRB enumeration. In the technique described in this paper, Postgate's B liquid medium was amended with 1.5 g l(-1) agar and non-SRB isolates were grown in the medium to create anaerobic conditions suitable for the growth of SRB. SRB counts were obtained within 2 to 7 days. For all tested non-SRB strains, most probable numbers (MPN) of SRB were 6 to 7 log higher than controls which were not preinoculated with non-SRB bacteria. Addition of agar improved MPN values significantly (P<0.05) after 2 days when preinoculated with Hafnia alvei TVA but exhibited no significant effect after 5 and 7 days. A significant effect of agar can be realized in those situations where the medium is agitated during transportation. When compared with liquid anaerobic media, this technique was significantly (P<0.05) faster and more sensitive in enumerating SRB and more so in those environmental samples which contained SRB in low numbers. The main advantage of this method is that it minimizes difficulties associated with using fastidious anaerobic techniques in enumerating SRB.