Microbicidal efficacy of superheated steam. II. Studies involving E. faecium and spores of B. xerothermodurans and B. coagulans

Abstract

AbstractThe authors' studies on the heat resistance of microorganisms and the superheating of steam (Spicher etal., Zbl. Hyg. Umweltmed. 201, 541 – 553, 1999) have been continued, using the vegetative bacterium, Enterococcus faecium, and spores of B. xerothermodurans and B. coagulans. The temperature of the saturated steam was 68 °C(E. faecium), 105 °C (B. xerothermodurans) and 110 °C (B. coagulans), respectively. The steam was superheated by 30 – 40K, as a maximum. The test organisms had been fixed to fibre glass fleece. The time of exposure to saturated or superheated steam after which 50% of the bioindicators yielded no viable germs was used as a measure of resistance of the organisms. In the discussion, reference has also been made to the findings for B. subtilis and B. stearothermophilus spores obtained in the preceding study.E. faecium exhibited a maximum resistance when superheating the steam by 5K. The superheated steam required a 74-fold exposure time compared to saturated steam of 68 °C. Resistance became gradually reduced as superheating was further increased. Even superheating by 30K required a 11-fold exposure time.The bacterial spores exhibited maxima of resistance on superheating by 23 – 30K. In these cases necessary exposure times were 119 (B. subtilis), 30 (B. xerothermodurans), and 4 times (B. coagulans, B. stearothermophilus) longer than those required in saturated steam. In the superheating range below 10K, behaviour patterns varied. Thus, heat resistance may initially become reduced with increasing superheating (B. coagulans), remain on almost the same level as under exposure to saturated steam (B. stearothermophilus), reach a stage of weakly enhanced resistance (B. xerothermodurans), or approach a maximum of resistance in an almost linear mode (B. subtilis). It appears that there are differences between strains of one and the same bacterial species.