Effect of some antibiotics on pigmentation in Serratia marcescens.

Serratia marcescens is characterized by its ability to produce a red pigment called prodigiosin. It is well known that there are some substances affecting pigmentation in bacteria. Cefoxitin, erythromycin, tobramycin, co-trimoxazole, imipenem and nitrofurantoin were found to have an inhibitory effect on pigmentation in a S. marcescens strain isolated from urine. It was also shown that the LD50 dose determined by inoculation of eight-week-old BALB/c mice with serial dilutions of overnight cultures of pigmented and nonpigmented variants was lower (LD50 = 300 x 10(3.94)) in the nonpigmented variant than in the pigmented one (LD50 = 300 x 10(5.58)). In addition, the Sereny test showed that in contrast to the pigmented variant, the nonpigmented variant caused keratitis in guinea pig eye.