Effect of Some Antibiotics on Pigmentation in Serratia marcescens

Summary 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 LD 50 dose determined by inoculation of eight-week-old BALE/c mice with serial dilutions of overnight cultures of pigmented and nonpigmented variants was lower (LD 50 = 300 × 10 3.94 ) in the nonpigmented variant than in the pigmented one (LD 50 = 300 × 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.