Letter to the editor: comments on a recent article by Burgos et al.

We wish to respond to the letter by Shryock et al. (2005) regarding our paper (Burgos et al., 2005). Their comments were, regrettably, inaccurate. First, Shryock and colleagues contended that we tested chloramphenicol, nalidixic acid, tetracycline, and penicillin because of their widespread use in dairy cattle. This is obviously incorrect, as some of these agents most certainly are not used in dairy cattle. What we did state was, “Four antibiotics were chosen (note new emphasis here) either because of their association with mar or their widespread use in dairy cattle” (Burgos et al., 2005). This clearly has a different meaning than that attributed by Shryock et al. (2005). Readers will note that bacterial resistance to chloramphenicol, nalidixic acid, and penicillin is increased when the mar locus is induced (Randall and Woodward, 2002), which was the deciding factor in choosing the antimicrobial agents for study. We agree with Shryock et al. (2005) about the agricultural use of tetracycline. In fact, tetracycline is commonly known to be extensively used as a growth enhancer in calves (Levy, 2002). This is why we chose to study tetracycline. Second, Shryock et al. (2005) stated that we employed nonstandard methods for the determination of MIC. This is again inaccurate. We wish to resolve this confusion by pointing out, for the benefit of the reader, that 1) a multitude of methods for MIC determinations exist (gradient plate, use dilution, E-test strip, Kirby-Bauer, disc diffusion, spiral, enzymatic, etc.), and 2) each of these methods is associated with its own protocol that is standardized, approved by the National Committee for Clinical Laboratory Standards, established, published, and routinely used by investigators. Therefore, it is reasonable to maintain that there is general acceptability of our gradient plate method (Koutsolioutsou et al., 2005) as well. As stated in our paper, we invoked the National Committee for Clinical Laboratory Standards (2002) method for MIC determination of aerobic bacteria from the Enterobacteriaceae family. The standardized protocols for the gradient plate method that we employed