Effect of certain antibacterial agents on the infrared spectrum of Micrococcus pyogenes var. aureus.

The report of Stevenson and Bolduan (Science, 116, 111, 1952) that species of bacteria could be differentiated by their infrared spectra suggested that this technique might be employed to study the effects of antibacterial agents on the organism. The preliminary experiments here reported were concerned with the effect of benzalkonium chloride (0.1 per cent) and phenol (5 per cent) upon the infrared spectrum of Micrococcus pyogenes var. aureus, strain 290. To 10 ml of a 22 to 26 hour nutrient broth culture of the test organism, 1 ml of the disinfectant was added, the mixture allowed to stand 10 minutes at room temperature, and then centrifuged. The sediment was spread evenly in a band about 20 mm wide over the diameter of the silver chloride plates and dried over anhydrous calcium sulfate. Thus, films of sufficient density to give from 80 to 90 per cent transmission at 5.50 , and little general light scattering, and yielding spectra easily reproduced in replicate experiments were obtained. A control, consisting of an untreated culture of the test organism handled in an identical manner, was run with each test. The Perkin-Elmer Model 21 infrared spectrophotometer was used for recording the spectra from 5.0 to 11.0,. By comparing the spectrum of the untreated organisms with that obtained after treatment with the antibacterial agent, it was observed that phenol markedly reduced the band at 7.17 to 7.20 u and had slight, if any, effect on the band occurring at 8.18 to 8.20 u. On the other hand, benzalkonium chloride (Roccal) obliterated the 7.17 , band and showed a more pronounced decrease in the intensity of the 8.18 pu band. In no instance was a new absorption band noted. These results indicate that antibacterial agents can affect selectively the infrared absorption bands of treated bacteria; hence the procedure may offer a useful tool for relating chemical structure to mechanism of activity. Experiments towards this are continuing.