Formation, Development, and Cross-Species Interactions in Biofilms

Abstract

Biofilms, which are essential vectors of bacterial survival, protect microbes from antibiotics and host immune attack and are one of the leading causes that maintain drug-resistant chronic infections. In nature, compared with monomicrobial biofilms, polymicrobial biofilms composed of multispecies bacteria predominate, which means that it is significant to explore the interactions between microorganisms from different kingdoms, species, and strains. Cross-microbial interactions exist during biofilm development, either synergistically or antagonistically. Although research into cross-species biofilms remains at an early stage, in this review, the important mechanisms that are involved in biofilm formation are delineated. Then, recent studies that investigated cross-species cooperation or synergy, competition or antagonism in biofilms, and various components that mediate those interactions will be elaborated. To determine approaches that minimize the harmful effects of biofilms, it is important to understand the interactions between microbial species. The knowledge gained from these investigations has the potential to guide studies into microbial sociality in natural settings and to help in the design of new medicines and therapies to treat bacterial infections.

Keywords: biofilm; cross-species interactions; horizontal gene transfer; metabolic interactions; quorum sensing.

Figure 1

Diagram of biofilm development. The development of a biofilm can be divided into six stages: planktonic bacteria, reversible attachment, irreversible attachment, microcolony, macrocolony, and dispersion. Planktonic bacteria attach to the surface through random or active movement, and the initial attachment is unstable and reversible. Contact with the surface promotes the stable and irreversible attachment of bacteria by contact-dependent gene expression. As the planktonic bacteria continue to attach and the attached bacteria multiply, microcolonies and macrocolonies that have complex three-dimensional structures gradually form. During this process, a series of phenotypic changes occur in the compactly distributed bacteria, which make the biofilm produce new ways to adapt to the environment. The typical macrocolonies are mushroom-like protrusions that are interspersed with fluid-filled water channels. In addition, macrocolonies establish more suitable shapes to adapt better to the environment. For example, in an aquatic environment with high flow rates, a biofilm can be flat or streamlined to buffer the high fluid shear force. Finally, some bacteria detach from the microcolony and disperse into the planktonic state, which initiates another new cycle of biofilm formation. Adapted with permission from Xin et al. (2010).

Figure 2

Five mechanisms for HGT. Conjugation is the HGT of bacteria by direct contact, and the DNA of the donor bacteria is transmitted to the recipient bacteria by the conjugative pili or adhesins. Transformation: the lysed bacteria (dashed line) release naked DNA fragments, which are acquired, integrated, and expressed by other bacteria. Transduction: a type of HGT mediated by bacteriophage. After the bacteriophage infects the bacteria, bacterial DNA fragments might be accidentally loaded into the bacteriophage head. The bacteriophage that carries the host bacteria’s DNA is released and infects a new bacterium to complete HGT. The bacteriophage with a red head represents that carries bacteriophage DNA and the blue head represents a bacteriophage that carries host bacterial DNA. Membrane vesicle fusion: the bacterial outer membrane bulges to form 20–250 nm MVs that carry genetic material and releases it into the environment. MVs have a lipid bilayer biological membrane that can protect and transport cargo and fuse with target cells to deliver contents. GTAs: bacteria that carry the GTA gene in chromosomes (brown fragment) can produce GTA. Most GTA particles carry a small random DNA fragment of the producing bacteria (blue particle head). A few GTA particles carry partial fragments of the GTA gene (brown particle head). Because these GTA gene fragments are not complete, they cannot be encoded into new GTA particles after being transferred into the recipient cell. GTA particles are released by bacteria cell lysis (dashed line).