# Anthrax-causing bacteria form biofilms on a soil-inspired porous glass bead model

## TL;DR

This study shows that anthrax-causing bacteria can form biofilms on a soil-like model, suggesting new environmental niches for their survival and spread.

## Contribution

The study demonstrates biofilm formation by Bcbva for the first time and suggests it may inhabit non-soil environments.

## Key findings

- Both B. anthracis and Bcbva form biofilms on a soil-inspired porous glass bead model.
- Bcbva shows distinct biofilm structures, hinting at alternative environmental niches like plant surfaces or water bodies.
- Biofilm formation may explain how these bacteria persist and transmit in the environment.

## Abstract

Bacillus anthracis and the emerging pathogen Bacillus cereus biovar anthracis (Bcbva) are causative agents of the lethal disease anthrax. Their ability to form highly resistant endospores enables them to persist in the environment, posing significant threats to wild herbivores, livestock, and also humans globally, especially considering their potential use as bioweapons. Despite the importance of these pathogens, the exact mechanisms underlying host infection remain poorly understood. Notably, both require relatively high infectious doses to cause disease. Proposed transmission routes include soil-based transmission and dissemination by carrion flies, which contaminate the area surrounding infected carcasses. However, considering the substantial dilution of bacteria and spores in these processes alongside the high infectious dose required, sustained transmission under natural conditions appears improbable. An alternative hypothesis is that B. anthracis and Bcbva can survive and proliferate in the environment by forming biofilms, structured bacterial communities attached to surfaces, which may serve as reservoirs for infection. While biofilm formation has been demonstrated for B. anthracis, data on Bcbva are lacking. To address this, we examined the biofilm-forming abilities of multiple B. anthracis and Bcbva strains using a three-dimensional, soil-inspired porous glass bead model (PGB) system. We applied confocal laser scanning microscopy, scanning electron microscopy, and quantitative cell counts to characterize biofilm development in detail. Our results confirm that both B. anthracis and Bcbva form biofilms under somewhat soil-like conditions. Furthermore, differences observed in biofilm structure and density between the above-mentioned species suggest that Bcbva may prefer additional environmental niches beyond soil, such as plant surfaces or small water bodies, potentially expanding our understanding of its environmental persistence and infection routes.

Anthrax is a deadly disease caused by bacteria that can form highly resistant spores, allowing them to survive in the environment for long periods. Two closely related bacteria, Bacillus anthracis and Bacillus cereus biovar anthracis (Bcbva), cause anthrax but differ in their natural habitats and likely in their modes of infecting animals and humans. Unfortunately, B. anthracis can also be misused as a biological weapon, giving it a grim reputation. While B. anthracis typically infects herbivores in grassland areas, Bcbva is mainly found in tropical rainforest environments, where its transmission and infection routes remain unclear. Our research investigates the possibility that both bacterial species can survive outside their hosts by forming biofilms, communities of bacteria that adhere to surfaces and protect themselves, thereby increasing the chance of infection. Using a soil-inspired laboratory model, we studied biofilm formation by these bacteria. Our findings confirm that both B. anthracis and Bcbva are capable of biofilm formation and suggest that Bcbva may also inhabit environments beyond soil, such as plant surfaces or small water bodies. Understanding these survival strategies is important for improving predictions of anthrax spread in nature and could aid in developing better methods for disease control and prevention, especially considering the potential misuse as a bioweapon.