increasing IPTG concentration (increasing levels of the PduA protein), and at concentrations >0.1I mM, primarily aberrant structures were observed (Figure 3 -6B). The fact that higher PduA expression levels alter organelle shape supports a structural role for the PduA protein in the formation of the structures. Strains with pduA Mutations Show a Period of "Interrupted" Growth when Cultured on PD/CN-B12 Minimal Medium

       During aerobic growth on PD/B 12/VJLT minimal medium, strain BE 182 (zlpduA) grew similarly to the wild type strain except for a period of "interrupted" growth that initiated at 15 h and persisted until the 40 h mark (Figure 3-7). Before the onset of "interrupted" growth, BE 182 (lJpduA) grew with a generation time of 6.8 hours which was slightly lower than the 8.4 h generation time of the wild type-strain. Strain BE 182 (*lduA) also grew faster than wild type after the period of "interrupted" growth; its generation time was 7.7 h compared to a 9 h generation time for the wild type strain. The maximum optical densities reached by both the wild type strain and BE1 82 (lJpduA) were 1.8 at 600 nm. Viable cell counts of the mutant and wild type strains gave results similar to optical density measurements. The period of "interrupted" growth was observed in four separate experiments. This phenomenon suggested that a toxic compound was accumulating and inhibiting the growth of S. enterica mutants unable to form polyhedral organelles during growth on PD. Presumably, growth resumed after the induction of genes that mitigated the toxicity problem. Alternatively, "interrupted" growth could have resulted from the depletion of an essential nutrient. However, this seems unlikely since "interrupted" growth was dependent on the concentration of PD and CN-B12 present in the growth medium (see below).