Role for σ38 in Prolonged Survival of Escherichia coli in Drosophila melanogaster

Abstract

Bacteria adapt themselves to host environments by altering the pattern of gene expression. The promoter-recognizing subunit σ of bacterial RNA polymerase plays a major role in the selection of genes to be transcribed. Among seven σ factors of Escherichia coli, σ38 is responsible for the transcription of genes in the stationary phase and under stressful conditions. We found a transient increase of σ38 when E. coli was injected into the hemocoel of Drosophila melanogaster. The loss of σ38 made E. coli rapidly eliminated in flies, and flies infected with σ38-lacking E. coli stayed alive longer than those infected with the parental strain. This was also observed in fly lines defective in humoral immune responses, but not in flies in which phagocytosis was impaired. The lack of σ38 did not influence the susceptibility of E. coli to phagocytosis, but made them vulnerable to killing after engulfment. The changes caused by the loss of σ38 were recovered by the forced expression of σ38-encoding rpoS as well as σ38-regulated katE and katG coding for enzymes that detoxify reactive oxygen species. These results collectively suggested that σ38 contributes to the prolonged survival of E. coli in Drosophila by inducing the production of enzymes that protect bacteria from killing in phagocytes. Considering the similarity in the mechanism of innate immunity against invading bacteria between fruit flies and humans, the products of these genes could be new targets for the development of more effective antibacterial remedies.

Inhibition of Phagocytic Killing of Escherichia coli in Drosophila Hemocytes by RNA Chaperone Hfq

“…All procedures were performed as described previously (41,44). In brief, male adult flies, 3-7 d after eclosion (15-20 flies per vial, one to three vials in each experiment), were anesthetized with CO 2 and abdominally injected with 50 nl of PBS containing a given number of bacteria (stated in each figure) with the aid of a nitrogen gas-operated microinjector (Narishige, Tokyo, Japan).…”

“…Once bound by bacterial substances, the receptors activate signaling pathways in immune cells to induce antibacterial reactions, including the production of antimicrobial peptides and the phagocytosis of bacteria (38)(39)(40). E. coli is generally considered to be nonpathogenic in Drosophila, but our previous study showed that a high load of E. coli may kill flies (41). Under such conditions, the Imd pathway, which is responsible for the production of antimicrobial peptides, and phagocytosis by hemocytes are required for the long survival of adult flies after infection with E. coli (41), suggesting the involvement of both humoral and cellular immune responses in the protection of Drosophila from pathogenic infection with E. coli.…”

“…Phagocytosis is a major cellular response among the self-defense strategies of Drosophila, which are employed to fight against infection with pathogenic bacteria, and its mechanism has been investigated in adult flies abdominally injected with bacteria, a surrogate procedure for septic infection (38,(41)(42)(43)(44)(45)(46). We previously reported that s 38 , a stress-responsive s factor of E. coli (47)(48)(49)(50)(51), is involved in the bacterial evasion of phagocytic killing in Drosophila (41).…”

“…s 38 -Encoding rpoS is included in the genes, the expression of which is controlled by Hfq (13,14,48,50,58). The expression of rpoS is transiently augmented when E. coli abdominally infects adult Drosophila (41). Anticipating the involvement of Hfq in s 38 actions to modulate the susceptibility of E. coli to phagocytosis, we conducted a series of biochemical and genetic experiments.…”

“…We previously reported that s 38 , a stress-responsive s factor of E. coli (47)(48)(49)(50)(51), is involved in the bacterial evasion of phagocytic killing in Drosophila (41). RNA polymerase of E. coli is a multisubunit enzyme consisting of a complex of four core subunits, a 2 bb9v, and a promoterrecognizing subunit named s (52)(53)(54).…”