# The initial inoculation ratio regulates bacterial coculture interactions and metabolic capacity

## TL;DR

This study shows how the starting ratio of two bacteria in a shared environment affects their interactions and ability to use different carbon sources.

## Contribution

The study reveals that initial inoculation ratios in bacterial cocultures influence final community structure and metabolic capacity under various carbon sources.

## Key findings

- The final bacterial ratio in cocultures depends heavily on the initial inoculation ratio in most carbon sources.
- Cocultures with initial ratios of 1:1 and 1000:1 showed high metabolic capacity on 14 specific carbon sources.
- Initial inoculation ratios can alter species interactions and induce emergent properties in microbial cocultures.

## Abstract

Coculture is an important model system in microbial ecology studies. As a key experimental parameter, the initial inoculation ratio has a crucial impact on the results of the coculture system. However, such an effect has never been investigated under multiple niche conditions. In this study, we established a simple coculture system with two model bacteria in various carbon sources and investigated the influence of initial inoculum ratios of 1:1000 to 1000:1 on community structure, function, and bacterial interaction. We found that the final ratio of the cocultures with different initial inoculum ratios differed in approximately five-sixths of the carbon sources, suggesting that the final ratio is highly dependent on the initial inoculum ratio, while the carbon source preferences of bacteria could not predict the final ratio of cocultures. Furthermore, we found that the initial ratio could regulate the metabolic capacity of the coculture, as only cocultures with initial ratios of 1:1 and 1000:1 gained high capacity on 14 specific carbon sources. The underlying reason may be that the pattern of species interaction is changed by the initial ratio. In conclusion, we showed that the initial ratio can induce emergent properties in coculture. These findings suggest that the initial ratio not only impacts the reproducibility of coculture experiments but also can influence our understanding of generic microbial ecology.