Conditional lethality of cell shape mutations of Salmonella typhimurium: rodA and mre mutants are lethal on solid but not in liquid medium

Round-cell (rodA, mre, divD) derivatives of a conditional alaS mutant of Salmonella typhimurium were studied under conditions allowing expression of tolerance to lethal cell shape mutations (41 degrees C), and under nontolerant conditions (30 degrees C). The rodA22::Tn10d(Kan) derivative grew normally (OD650 nm) in LB-broth at 30 degrees C; however, doubling of total cell count took much longer (130 min) than at 41 degrees C (57 min). Although the cells were able to divide in LB-broth at 30 degrees C, viable count on LB-agar at 30 degrees C was 10(3)-fold lower than on LB-agar at 41 degrees C. Phase-contrast microscopy of rodA cells incubated under different conditions showed that their size increased on LB-soft agar at 30 degrees C, but they failed to divide and finally lysed. In contrast, division occurred in LB-broth at 30 degrees C and also in LB-broth and LB-soft agar at 41 degrees C. The mre-17::Tn10d(Kan) derivative acted like the rodA strain whereas the divD135::Tn10d(Kan) mutant behaved normally both at 30 degrees C and 41 degrees C. It is concluded that rodA and mre mutations delay cell division, but are lethal only on solid medium. Mutations conferring tolerance to "lethal" rodA and mre mutations improve division performance both in liquid and solid media.