# Pedal effect: aerobic and facultative anaerobic Enterobacter hormaechei maintain the homeostasis of the house fly gut microbiota under different oxygen conditions

## TL;DR

This study shows how aerobic and facultative anaerobic bacteria in the house fly gut work together to support larval growth and immune function under different oxygen conditions.

## Contribution

The study introduces the 'pedal effect' concept, revealing how aerobic and anaerobic bacteria interact to maintain gut microbiota stability in house fly larvae.

## Key findings

- Aerobic and facultative anaerobic E. hormaechei strains promote larval growth and immune activity when added to the diet.
- Phage treatments reduced beneficial bacteria and increased harmful ones, showing their regulatory role in gut microbiota.
- EhX and EhY dominate midgut and hindgut, respectively, forming a competitive-cooperative balance in the gut.

## Abstract

House fly gut microbiota plays a major role in regulating larval development and immune responses. As the house fly gut microbiota is complex with different functions, the role of many gut microbiota in the host remains unknown. The differential oxygen content in the midgut and hindgut of house fly intestines creates distinct gut microenvironments, resulting in variations in the composition of gut microbiota. Research on the distribution of aerobic and facultative anaerobic bacteria in different parts of the intestine—and how they interact with each other—is helpful for exploring the interaction between intestinal bacteria and the host in aerobic and anaerobic states. However, there have been no mechanistic studies on the interactions of gut bacteria in different oxygen microenvironments within the house fly.

In this study, aerobic Enterobacter hormaechei EhX and facultative anaerobic E. hormaechei EhY were isolated from the guts of house fly larvae. The specific phages, EhXP and EhYP, were isolated against these two bacteria, with phage EhXP and EhYP specifically targeting EhX and EhY, respectively. Furthermore, the role of the aerobic–anaerobic interaction in E. hormaechei during larval development was investigated using phage and bacterial feeding experiments.

The results showed that the addition of aerobic E. hormaechei EhX or facultative anaerobic E. hormaechei EhY alone to the diet promoted larval growth and phenoloxidase activity. However, in the phage-treated groups, the opposite effect was observed. In contrast, EhY was the dominant bacteria in larval hindgut. Conversely, EhX was the main bacteria in the midgut of larvae. Aerobic/facultative anaerobic gut bacteria competed and cooperated to maintain a stable composition of larval gut microbiota, forming a negative correlational “pedal effect” promoting larval development. Through high-throughput sequencing it was demonstrated that the abundance of beneficial bacteria increased in the bacteria treatment group. The number of harmful bacteria increased in the phage treatment group.

The results revealed the crucial role of aerobic and facultative anaerobic E. hormaechei in the house fly. These results highlight the potential of future applications of multiple phages for the study of insect intestinal flora.

The online version contains supplementary material available at 10.1186/s13071-025-07013-5.