Sniffing Bacterial Cultures on Agar Plates: a Useful Tool or a Safety Hazard?

There has recently been a flurry of e-mails on the ASM DivC list concerning the safety of sniffing bacterial cultures. Opinions with respect to the safety of sniffing were divided. Several contributors were adamant that all sniffing should be prohibited. Others were skeptical and countered that the risk was very small and that sniffing was a useful tool. The undoubted reality of laboratory-acquired infections was considered to stem from manipulating and producing aerosols of pathogenic organisms outside of a biological safety cabinet, although some felt that they had witnessed cases where the only exposure was from sniffing plates. This seems unlikely, as almost any further processing of plate cultures is likely to involve the production of aerosols. This letter reports our attempt to measure the bacterial load of the air above bacterial cultures on solid agar.

Fresh overnight cultures on blood agar plates of nine genera of organisms commonly isolated in our laboratory were prepared. Escherichia coli was chosen to represent Salmonella enterica serovar Typhi. A Brucella sp. was not tested as we felt the risk was not warranted. We removed the lids and sampled the air above the plates for 4 min from a distance of 2 cm with a Biotest HYCON standard RCS air sampler that has a separation volume of 40 liters/min. The agar strips were incubated at 35°C for 48 h in an appropriate atmosphere, and the colonies were identified by traditional methods. The results are shown in Table 1.

TABLE 1.

Colony count from air sampled 2 cm from cultures on blood agar

Organism	Colony count	No. of CFU/m3
Neisseria meningitidis	0	0
Streptococcus pneumoniae	0	0
Burkholderia pseudomallei	1	6.25
Bacillus spp.	1	6.25
Pseudomonas aeruginosa	1	6.25
Staphylococcus aureus	2	12.5
Escherichia coli	1	6.25
Candida albicans	0	0
Bacteroides fragilis	0	0

The highest colony count was equal to 12.5 CFU/m³. An average sniff might be expected to sample 50 to 200 ml of air, so it would require 6,000 to 7,000 sniffs to inhale 12.5 CFU. The velocity of air while sniffing might be expected to vary and to affect the number of cells drawn off the colonies. Depending on the enthusiasm of the sniffer, it might well exceed that produced by the air sampler. Nevertheless, in our experience the lid is only partially removed and sniffing is conducted with caution, and the air velocity is low. Technique is relevant: the aim of plate-sniffing is to detect particular volatile compounds (for example, diacetyl produced by members of the anginosus group of streptococci [1]). It should be possible by opening a plate gently to allow outward diffusion of such moieties without allowing rapid air currents to move across the surface of the agar.

Our results suggest that the risk of inhaling a significant dose of a bacterium from solid agar is low. Despite this, we would caution staff against sniffing plate cultures that they suspect to include Neisseria meningitidis, Brucella spp., or other highly virulent pathogens.