Selection ofStaphylococcus aureusin cultures from air samples

Abstract

Although a good many selective culture media have been described for the isolation of Staphylococcus aureus, none of them has proved useful for the examination of cultures from air. This is partly because, in examining such cultures, one is trying to select the S. aureus from among a great variety of other members of the Micrococcaceae. Media depending on a high concentration of sodium chloride (e.g. Chapman, 1944), which are principally inhibitory for enterobacteria and similar Gram-negative bacilli, are not, therefore, of much use. Other media, such as those containing potassium tellurite (Zebovitz, Evans & Niven, 1955) or polymyxin B (Davis & Davis, 1965), though often satisfactory for examination of material from infected lesions or carrier sites, prove too inhibitory for the isolation of staphylococci from air. It has been a common experience that bacteria are often difficult to cultivate when collected from the airborne state. The recent studies on the taxonomy of staphylococci and micrococci published by Baird-Parker (1963, 1965) suggest a new approach, for he showed that S. aureus is able to grow at a higher temperature than most of the micrococci. Moreover members of the genus Staphylococcus are able to grow under anaerobic conditions, while those of the genus Micrococcus cannot, and other studies have shown that a substantial proportion of the airborne cocci fall into Micrococcus (Corse & Williams, 1968). We therefore explored the use of anaerobic culture at 41°C. as a selective method for the isolation of S. aureus. After a number of preliminary experiments with pure cultures, which need not be discussed here, all our studies were made by the exposure of culture plates containing the media under test in the wards of St Mary's Hospital. METHODS Air samplingCulture plates 14 cm. in diameter were exposed in hospital wards for periods varying between 8 and 16 hr. The plates were stacked in the holder illustrated in Plate 1, which provides about 5 cm. separation between plates. Preliminary experiments showed that there were no significant differences in the numbers of colonies developing on the four plates in the stack. The culture plates with the different media were placed at random on the four layers of the stack. One or more stacks of plates were exposed each day but each stack included at least one 'control' plate.The culture media used are listed in Table 1. 3-2

Studies on the microbial flora in the air of submarines and the nasopharyngeal flora of the crew

“…aureus using various selective media (Harding & Williams, 1969) Nasopharyngeal swabs for total and differential colony counts of bacteria During the studies on meningococcal carrier rates, large numbers of gramnegative rods, chiefly of the Enterobacteriaceae but also many pseudomonads, were isolated from nasopharyngeal swabs. Semi-quantitative studies were therefore carried out to determine the extent, frequency and duration of throat carriage of these organisms.…”

Selective Media in Air Sampling: A Review

“…It must be remembered that colonies from damaged organisms may develop slowly and appear atypical. For environmental use in air sampling, it is probable that only 2 selective methods have been properly tested, that of Williams & Hirch (1950) for streptococci, in a medium which is not completely satisfactory, and that of Harding & Williams (1969) for Staph. aureus, which was tested only for use with settle plates.…”

“…Most medical bacteriologists tend to incubate a t 37" automatically, so that many figures for the abundance of airborne flora refer to organisms growing at that temperature, usually after overnight aerobic incubation. Apart from the paper of Harding & Williams (1969) discussed in the previous section, little work has been done on temperature of incubation as a selective agent for airborne bacteria. Kotula & Kinner (1964), who used an Andersen sampler, had a category of psychrophilic organisms, i.e.…”

“…aureus to grow (Kingston, unpublished). However, Harding & Williams (1969) found that polymyxin (concentration unstated) was too inhibitory. I n looking for Gram negative rods, 10-50 mg/l of methicillin or cloxacillin should eliminate virtually all the Gram positive organisms except perhaps Strep.…”