Passage
The lac operon encodes the proteins that metabolize lactose and is a key biochemical feature that differentiates between members of the Enterobacteriaceae family. For example, the severely virulent Salmonella typhimurium can be distinguished from its less virulent relative Escherichia coli by the absence of a lac operon. Researchers propose that the evolutionary loss of the lac operon may contribute to S. typhimurium's ability to invade epithelial cells.To test the hypothesis, wild-type (WT) S. typhimurium cells were transformed with plasmids encoding components of the lac operon (Figure 1) : β-galactosidase (lacZ⁺) , lactose permease (lacY⁺) , transacetylase (lacA⁺) , or all three (lac⁺) . Each plasmid also contained genes for the lac repressor (lacI) , which can inhibit lac operon transcription in the absence of lactose; the cAMP receptor protein (crp) , which induces lac expression in low glucose; and a chloramphenicol resistance gene (cat) .
Figure 1 Plasmid PYM-LACZYA (lac⁺) , used to transform WT S. typhimurium. Similar plasmids containing only one of the three lacZ, lacY, or lacA genes were also used.Virulence assays were performed in mice by inoculating with WT or lac⁺ S. typhimurium. The assays confirmed that lac⁺ bacteria were less virulent than their WT counterparts, and that both WT and lac⁺ bacteria were more virulent in the presence of glucose. Subsequently, several genes related to invasion were analyzed for their expression levels in the presence or absence of lac plasmids (Table 1; Figure 2) .Table 1 Transcription Levels of Invasion-Associated Genes in lac⁺ S. typhimurium Relative to WT
Figure 2 Transcript concentrations of two S. typhimurium flagellar genes, fljB and fliC, in bacteria transformed with lacZ⁺, lacY⁺, lacA⁺, or lac⁺ plasmids. Concentrations are expressed as a percentage of WT levels.
Adapted from Jiang L, Ni Z, Wang L, Feng L, Liu B. Loss of the lac operon contributes to Salmonella invasion of epithelial cells through derepression of flagellar synthesis. Curr Microbiol. 2015;70(3) :315-23.
-Which of the following conclusions regarding virulence of lac⁺ S. typhimurium is most accurate?

Question

Passage
The lac operon encodes the proteins that metabolize lactose and is a key biochemical feature that differentiates between members of the Enterobacteriaceae family. For example, the severely virulent Salmonella typhimurium can be distinguished from its less virulent relative Escherichia coli by the absence of a lac operon. Researchers propose that the evolutionary loss of the lac operon may contribute to S. typhimurium's ability to invade epithelial cells.To test the hypothesis, wild-type (WT) S. typhimurium cells were transformed with plasmids encoding components of the lac operon (Figure 1) : β-galactosidase (lacZ⁺) , lactose permease (lacY⁺) , transacetylase (lacA⁺) , or all three (lac⁺) . Each plasmid also contained genes for the lac repressor (lacI) , which can inhibit lac operon transcription in the absence of lactose; the cAMP receptor protein (crp) , which induces lac expression in low glucose; and a chloramphenicol resistance gene (cat) .

Figure 1 Plasmid PYM-LACZYA (lac⁺) , used to transform WT S. typhimurium. Similar plasmids containing only one of the three lacZ, lacY, or lacA genes were also used.Virulence assays were performed in mice by inoculating with WT or lac⁺ S. typhimurium. The assays confirmed that lac⁺ bacteria were less virulent than their WT counterparts, and that both WT and lac⁺ bacteria were more virulent in the presence of glucose. Subsequently, several genes related to invasion were analyzed for their expression levels in the presence or absence of lac plasmids (Table 1; Figure 2) .Table 1 Transcription Levels of Invasion-Associated Genes in lac⁺ S. typhimurium Relative to WT

Figure 2 Transcript concentrations of two S. typhimurium flagellar genes, fljB and fliC, in bacteria transformed with lacZ⁺, lacY⁺, lacA⁺, or lac⁺ plasmids. Concentrations are expressed as a percentage of WT levels.
Adapted from Jiang L, Ni Z, Wang L, Feng L, Liu B. Loss of the lac operon contributes to Salmonella invasion of epithelial cells through derepression of flagellar synthesis. Curr Microbiol. 2015;70(3) :315-23.
-Which of the following conclusions regarding virulence of lac⁺ S. typhimurium is most accurate?

Quizplus · Accepted Answer

11ecba2d_0f66_26be_a3bf_b57588d27f35_MD0008_00 Chemotaxis is the process by which mobile cells both sense and move toward or away from an increasing concentration of a specific chemical. Many pathogens find host cells to invade by sensing the chemicals that they release and moving toward them. Impaired ability to either sense or respond to chemical signals can reduce virulence. Table 1 illustrates the differences between WT and lac⁺ bacterial expression of several genes involved in invasion of host cells. A p-value indicates the probability that an observed change did not occur due to random chance, with p < 0.05 generally being considered statistically significant. Based on the information in the table, the only genes with a significant decrease in expression level (p < 0.05) are the flagellar genes (fljB and fliC) that code for proteins in the flagellum of S. typhimurium. The chemosensation genes are not significantly affected by the presence of the lac genes, so the bacteria are likely still able to detect the chemical signals produced by epithelial cells. However, impaired flagella would inhibit motion toward the signal's source. (Choice A) The transcription of chemosensation genes is not significantly affected by the lac operon (p > 0.05), so S. typhimurium bacteria are most likely able to detect epithelial cells adequately. (Choices B and C) lac⁺ S. typhimurium bacteria show no significant change in the expression of type III secretion genes. Educational objective: Bacteria respond to chemical stimuli in the extracellular environment by moving away from or toward increasing concentrations of signal molecules. This movement, known as chemotaxis, allows bacteria to adjust the direction of their movement toward target cells and is an important part of infection. Impairing the ability to detect or respond to chemical signals can reduce virulence.

Passage the Lac Operon Encodes the Proteins That Metabolize Lactose and and Is