Essential gene Gale Mucoid mutant Salmonella enterica Salmonella enterica subsp. enterica serovar Typhimurium Typhimurium article bacterial gene bacterial genome bacterium colony colanic acid controlled study gene cassette gene expression gene function gene inactivation gene insertion gene mutation gene product genetic analysis genetic complementation genetic variability igaA kanamycin lethality molecular cloning mucM nonhuman null allele open reading frame