Extragenic suppressors of growth defects in msbB Salmonella

Lipid A, a potent endotoxin which can cause septic shock, anchors lipopolys accharide (LPS) into the outer leaflet of the outer membrane of gram-negati ve bacteria. MsbB acylates (KDO)(2)-(lauroyl)-lipid IV-A with myristate dur ing lipid A biosynthesis. Reports of knockouts of the msbB gene describe ef fects on virulence but describe no evidence of growth defects in Escherichi a coli K-12 or Salmonella. Our data confirm the general lack of growth defe cts in msbB E. coli K-12. In contrast, msbB Salmonella enterica serovar Typ himurium exhibits marked sensitivity to galactose-MacConkey and 6 mM EGTA m edia. At 37 degreesC in Luria-Bertani (LB) broth, msbB Salmonella cells elo ngate, form bulges, and grow slowly. msbB Salmonella grow well on LB-no sal t (LB-0) agar; however, under specific shaking conditions in LB-0 broth, ma ny msbB Salmonella cells lyse during exponential growth and a fraction of t he cells form filaments. msbB Salmonella grow with a near-wild-type growth rate in MSB (LB-0 containing Mg2+ and Ca2+) broth (23 to 42 degreesC). Extr agenic compensatory mutations, which partially suppress the growth defects, spontaneously occur at high frequency, and mutants can be isolated on medi a selective for faster growing derivatives. One of the suppressor mutations maps at 19.8 centisomes and is a recessive IS10 insertional mutation in so mA, a gene of unknown function which corresponds to ybjX in E. coli. In add ition, random TWO mutagenesis carried out in an unsuppressed msbB strain pr oduced a set of TWO inserts, not in msbB or somA, that correlate with diffe rent suppressor phenotypes. Thus, insertional mutations, in somA and other genes, can suppress the msbB phenotype.