Detection of Heated Bacterial Spores with Fluid Thioglycollate and Soybean Casein Digest Broths Containing Variable Concentrations of Solids

Abstract

Spore suspensions of Bacillus stearothermophilus (ATCC 7953, 10149, 12980), Bacillus subtilis (ATCC 6633), Bacillus subtilis var. niger (ATCC 9372), Bacillus pumilus (ATCC 27142, and a wild strain) and Bacillus cereus (ATCC 11778) were heated (20 min at 121°C for thermophiles, and 2.5–20 min at 100°C, 105°C, or 107°C for mesophiles) in 1 ml sealed ampoules. Surviving spores were recovered by a 5-tube most probable number (MPN) procedure with soybean casein digest (SCD) or fluid thioglycollate (FTH) broths containing variable concentrations of solids. Numbers of heated thermophilic spores (ATCC 7953) recovered with SCD broths were approximately 1.5 log MPN/ml higher at the 100% broth solids concentration than numbers recovered with FTH broth. Increasing the solids of SCD broth to twice the recommended amount (200%) reduced recovery of heated thermophilic spores compared to regular (100%) strength broth. Recovery of heated thermophilic spores with SCD or FTH broths was higher (P < 0.05) when their content of solids was reduced to as low as 0.391% compared to regular concentrations (100%). Broths with concentrations of solids at 10% of the manufacturer's recommended levels resulted in maximum recovery of heated thermophilic spores. In contrast, reduction of solids in SCD broth to 10% did not (P > 0.05) influence recovery of heated mesophilic spores compared to SCD broth with regular concentration of solids. Broths with reduced as well as regular concentrations of solids were equally effective in recovering unheated spores. These results should be useful in future modifications of methodology for sterility testing.