Introduction
The study of microbial biofilms has evolved significantly since their initial discovery nearly 40 years ago. Biofilms, which are structured communities of microbial cells embedded in a self-produced extracellular matrix, have become a central focus in understanding chronic infections and their resistance to treatment. This is particularly relevant in the context of the Bartonella genus, which includes several species associated with various human diseases. Bartonella henselae, one of the most studied species, is known to cause cat scratch disease (CSD), and its ability to form biofilms has been linked to chronic and difficult-to-treat infections.
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Bartonella henselae is a gram-negative, intracellular zoonotic bacterium that primarily infects cats, which serve as its natural reservoir. While cats often exhibit no symptoms, Bartonella henselae infection can cause a range of clinical issues in both cats and humans, including endocarditis, ocular diseases, and neurological symptoms. Human infections typically occur through scratches or bites from infected cats, but other modes of transmission, such as flea bites, have been documented. The growing concern about flea infestations and increasing resistance to flea control measures have highlighted the need for a deeper understanding of Bartonella henselae's persistence mechanisms, particularly its ability to form biofilms.
Fig. 1 Scanning electron micrograph of a 48 h Bartonella henselae biofilm growing on a 3-dimensional nanofibrous scaffold. (Okaro U., et al. 2021)
Clinical Importance
Bartonella henselae is associated with several clinical manifestations, including cat scratch disease (CSD), which is characterized by lymphadenopathy, fever, and chronic lymphadenopathy. CSD is predominantly reported in children and is often accompanied by persistent bacteremia. More severe manifestations of Bartonella henselae infection include bacillary angiomatosis, peliosis hepatitis, neurological conditions, and life-threatening infective endocarditis. The complexity of Bartonella henselae infections is further compounded by its ability to evade the host immune system and resist antimicrobial treatment.
Fleas, particularly Ctenocephalides felis, are known vectors for Bartonella henselae. Infected fleas transmit the bacteria to cats, which then serve as reservoirs for human infections. The clinical challenge in treating Bartonella henselae infections is exacerbated by the bacterium's ability to form biofilms. Biofilms are structured communities of bacterial cells surrounded by a protective extracellular matrix, which includes proteins, polysaccharides, and extracellular DNA (eDNA). This matrix provides a barrier against antibiotic penetration, making biofilm-associated infections difficult to treat. Bartonella henselae biofilms are implicated in chronic infections and are associated with increased antibiotic resistance.
Biofilm Formation, Composition, and Life Cycle
Biofilm formation is a complex process that begins with bacterial adhesion to a surface. Bacteria can grow as free-floating planktonic cells or as aggregates embedded in a biofilm matrix. In the case of Bartonella henselae, the initial adhesion is mediated by surface adhesins such as BadA. The formation of a biofilm involves several stages: initial adhesion, aggregation, maturation, and dispersal. Bartonella henselae biofilms have been shown to contain proteins, polysaccharides, and eDNA, all of which contribute to the stability and resistance of the biofilm.
The role of BadA, a trimeric autotransporter adhesin, has been well-documented in Bartonella henselae. BadA is essential for the bacterium's adhesion to host cells and extracellular matrix proteins. Mutants lacking BadA fail to form stable biofilms, highlighting its critical role in biofilm formation. In addition to BadA, other factors such as the type IV secretion system (T4SS) and various regulatory proteins also play roles in biofilm formation and bacterial persistence.
In humans, Bartonella henselae is known to cause a range of diseases that require biofilm formation for persistent infection. For example, infective endocarditis, a severe and difficult-to-treat condition, is often associated with Bartonella henselae biofilms. The ability of Bartonella henselae to form biofilms in both the flea and mammalian hosts contributes to its persistence and pathogenicity.
Genetic Regulation of Biofilm Formation
Biofilm formation in bacteria is regulated by a variety of genetic and environmental factors. In many bacterial species, cyclic diguanosine monophosphate (c-di-GMP), small RNAs (sRNAs), and quorum sensing (QS) play critical roles in biofilm regulation. In Bartonella henselae, biofilm formation is influenced by several genetic elements, including sRNAs and transcription factors.
One key regulatory element in Bartonella henselae is the Bartonella regulatory transcripts (Brts), a family of sRNAs that play a role in biofilm formation. These sRNAs are highly conserved and transcribed at high levels, suggesting their importance in bacterial regulation. The Brt sRNAs are thought to function as riboswitches or RNA thermometers, which regulate downstream genes in response to environmental conditions. For example, the Brt1 sRNA has been shown to interact with mRNAs coding for surface adhesins and transcription factors, influencing biofilm formation.
In addition to sRNAs, the Trp proteins in Bartonella henselae are also involved in biofilm regulation. Trp1, a transcription factor, binds to the promoter region of the badA gene, regulating its expression and influencing biofilm formation. The expression of Trp1 and other Trp proteins is upregulated in biofilm cells, suggesting their role in biofilm development.
The regulatory network controlling biofilm formation in Bartonella henselae is complex and involves multiple layers of regulation. The interplay between sRNAs, transcription factors, and environmental conditions determines the bacterium's ability to form and maintain biofilms.
Environmental and Host Factors Influencing Biofilm Formation
The formation and persistence of Bartonella henselae biofilms are influenced by various environmental and host factors. In the flea host, Bartonella henselae forms biofilms in the gut and fecal matter, which helps the bacteria survive and replicate. The presence of heme in the flea gut and feces affects biofilm formation, with high heme concentrations influencing gene expression and biofilm stability.
In mammalian hosts, biofilms contribute to chronic infections and resistance to treatment. Bartonella henselae biofilms are resistant to antibiotics and host immune responses, making infections difficult to treat. The ability of Bartonella henselae to form biofilms in different environments, including the flea and mammalian hosts, underscores its adaptability and persistence.
Conclusion
Bartonella henselae's ability to form biofilms plays a crucial role in its pathogenicity and persistence in both vector and host environments. The complex interplay between genetic regulation, environmental conditions, and host factors influences biofilm formation and contributes to the difficulty of treating Bartonella henselae infections. Understanding the mechanisms of biofilm formation and regulation in Bartonella henselae is essential for developing more effective treatments and addressing the challenges associated with chronic and difficult-to-treat infections. Continued research into the genetic and environmental factors influencing biofilm formation will provide valuable insights into the pathogenesis of Bartonella henselae and other biofilm-forming bacteria.
References
- Okaro U., et al. What is in a cat scratch? Growth of bartonella henselae in a biofilm. Microorganisms. 2021, 9 (4): 835.
- Raghunathan D., et al. SadA, a trimeric autotransporter from Salmonella enterica serovar Typhimurium, can promote biofilm formation and provides limited protection against infection. Infection and Immunity. 2011, 79 (11): 4342-4352.
- Riess T., et al. Analysis of Bartonella adhesin A expression reveals differences between various B. henselae strains. Infection and Immunity. 2007, 75 (1): 35-43.
- Okaro U., et al. A non-coding RNA controls transcription of a gene encoding a DNA binding protein that modulates biofilm development in Bartonella henselae. Microbial Pathogenesis. 2020, 147: 104272.
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