Development of a laboratory autoclave system

Abstract

The requirements of industry, the National Health Service, and in particular the published work of Godber and Howie (DHSS, 1975, 1978) have produced a demand for the reassessment of autoclave standards. It has long been appreciated by those responsible for laboratory autoclaves that, with the cycles used, some loads are unlikely to reach and maintain the temperature required to kill the more resistant bacterial spores (Gillespie and Gibbons, 1975 ; Public Health Laboratory Service Subcommittee, 1978), and discard cycles of several hours duration may be required to guarantee sterilization. However, failure to reach sterilization conditions has often been considered unimportant since the discarded material usually consists of enriched culture media in which the spore-bearing bacteria revert to their heat-sensitive vegetative form, and most spores are unlikely to cause infection in healthy individuals. The need for a prolonged sterilization time is usually due to poor air removal and inadequate controls (Everall, Morris and Yarnell, 1978). Laboratory autoclaves are often little more than enlarged pressure cookers with air removal by upward or downward displacement unassisted by vacuum extraction, and the chamber is rarely jacketted. Temperature and pressure are usually controlled by a hand-operated valve and instrumentation limited to a single temperature and pressure gauge. Such systems were chosen to give the flexibility required for a wide range of loads, including heat-sensitive media which is readily damaged by excess temperature and prolonged times above 100°C. Figure 1 shows a typical cycle from a downward displacement unjacketted autoclave, Many such autoclaves are still in use in laboratories, and although there is no evidence that their failure to meet current requirements has been associated with infections, modern health and safety requirements described in the Howie Report have led the PHLS Subcommittee (1981) to write a standard for an improved laboratory autoclave.