For schools that are unfamiliar with the variety of items required for practical studies of microbes, click on the Practical activities page of the MiSAC web site and download Practical Microbiology for Secondary Schools. Scanning through its 21 investigations will provide a good introduction to the equipment that you may require.

Further microbiological guidance to schools is also available to members from CLEAPSS www.cleapss.org.uk and SSERC www.sserc.org.uk. For example, CLEAPSS has compiled GL283, a list of essential and suggested items of equipment for microbiology investigations, together with an Excel file of prices. In Scotland, SSERC provides information for useful equipment: www.sserc.org.uk/health-safety/biology-health-safety/microbiological-techniques/.

Sterilisation equipment

A pressure cooker or autoclave is essential for preparing sterile media, instruments and glassware and to sterilise used agar plates and cultures before disposal. A Prestige 6 litre, hi-dome pressure cooker + gas ring or hotplate is probably the best sterilising equipment for introductory microbiology investigations. The deeper lid provides much-needed height for sterilising media in glassware; it is best to use medical flat bottles. These enable greater volumes of media to be prepared than using conical flasks and their flat shape makes it easier for heat to penetrate the liquid for sterilisation to be achieved. The hi-dome model's capacity is 6 litres but, for preparing larger quantities of media and sterilising used agar plates before their disposal, this volume soon becomes frustratingly limited, and a larger pressure vessel will be preferable.

A range of larger pressure cookers is available; several items are listed on Amazon.co.uk, including models such as the Hawkins BigBoys which have capacities of 14, 18 or 22 litres. It is most important to check the pressure at which larger pressure cookers operate; this should be at 103 kPa / 15 psi to produce the required temperature of 121°C for 15 minutes to ensure sterilisation is achieved. Some models of larger pressure cookers operate at lower pressures than 103 kPa. The design of the lid of the Hawkins BigBoy models makes it quite tricky to fit to ensure the gasket is correctly seated. Nevertheless, these models perform satisfactorily, enabling their contents to be sterilised after 15 minutes, once the operational pressure is reached; the natural venting of steam during this period can, however, be somewhat alarming! One problem is the extended time that it takes to heat the larger-capacity models before operational pressure is approached, followed by the free-steaming period needed so that all the air is vented to ensure only steam fills the pressure vessel. Unless a 22-litre capacity is essential for the scale of practical work that is planned, the smaller BigBoy models may be the preferred option.

Unfortunately, the requirements of the Pressure Systems Safety Regulations (PSSR) have a major influence for microbiology practical investigations. Any vessel containing steam at raised pressures must be inspected to ensure that it is safe. The PSSR requires that a certified written scheme of examination (WSE) has to be obtained before the equipment is first operated. This scheme is then used by a competent person to check that the equipment is safe. For a pressure cooker or autoclave which does not involve thermostatically-controlled temperature regulation, a teacher or technician could be the competent person who uses a certified WSE to inspect the equipment. Autoclaves with thermostatically-controlled heating will require the services of an engineer for their inspection and this will be very expensive and recurs every year.

Guidance to schools on how the health & safety obligations of the PSSR can be fulfilled is available to members of CLEAPSS: See G214a Examining Autoclaves, Pressure Cookers and Model Steam Engines: Guidance for employers, GL249 Examination of pressure vessels in schools - suppliers guide and also a video: An introduction to the Pressure Systems Safety Regulations (PSSR) in schools. In addition, there is a considerable amount of further related information, such as GL168 Guidance on safe use & maintenance of pressure cookers and autoclaves, GL126 Portable autoclave and pressure cooker buying guide and GL272 Sterilising for microbiology.

SSERC can also advise member schools in Scotland. For legislation around PSSR, see www.sserc.org.uk/wp-content/uploads/2017/03/PSSR.docx.

For schools applying for a grant to purchase sterilising equipment, as well as stating the price of the equipment (including gas ring or hot plate), the cost of obtaining a certified WSE and organising its periodic inspection to meet the requirements of the Pressure Systems Safety Regulations (PSSR) must be included.