Effect of some antibiotics on pigmentation in Serratia marcescens

Serratia marcescens is characterized by its ability to produce a red pigmen t called prodigiosin. It is well known that there are some substances affec ting pigmentation in bacteria. Cefoxitin, erythromycin, tobramycin, co-trim oxazole, imipenem and nitrofurantoin were found to have an inhibitory effec t on pigmentation in a S. marcescens strain isolated from urine. it was als o shown that the LD50 dose determined by inoculation of eight-week-old BALB /c mice with serial dilutions of overnight cultures of pigmented and nonpig mented variants was lower (LD50 = 300 X 10(3.94)) in the nonpigmented varia nt than in the pigmented one (LD50 = 300 x 10(5.58)). In addition, the Sere ny test showed that in contrast to the pigmented variant, the nonpigmented variant caused keratitis in guinea pig eye.