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Bartonella species are vector-borne zoonotic bacteria. Let’s unpack that. “Vector-borne” means the bacteria can be transferred between hosts via a vector such as a flea. “Zoonotic” means that it can survive in an animal reservoir and then return to humans.
The plus side for the Bartonella species, or any other zoonotic bacteria trying to survive in a hard world, is that even if it is totally wiped out in humans and their homes, it can survive in a reservoir species until it has another opportunity to jump back to a human host. And being vector-borne gives it the opportunity to move through the environment between mammalian hosts.
But there is also a plus side for the humans trying to avoid these bacteria or kill them if they are infected. Vector-borne zoonotic bacteria have to jump through a lot of hoops to make all of this work. Contradictory evolutionary pressures mean they can be good at living in each host and vector, but they might not ever be great at it.
There are a lot of bacteria that aren’t great at infecting humans. For example, Capnocytophaga species are usually present in dogs and cats, but they almost never infect humans because they are bad at it. Recent research suggests that in order to infect humans, the human must have a hereditary trait that results in a specific weakness in his or her immune system. What if researchers could find out why Bartonella is more successful at infecting humans and come up with a treatment to overcome it?
One of the reasons Bartonella is successful may be gene duplication. Research on Bartonella henselae found that it has several copies of genes that provide the same function to the cell. This may seem redundant, but it provides an evolutionary advantage. The same genes are able to evolve independently of each other and in slightly different ways over time.
Researchers have speculated that gene duplication might be part of how Bartonella henselae has become so successful at living in a variety of hosts and vectors. Studies of other types of microbes have found that gene duplication allows a microbe to develop antibiotic resistance from one copy of the gene while still retaining the original function in another copy of the gene.
A specific example of how gene duplication works in Bartonella species comes from the research on a variety of genes that are responsible for heme-binding proteins found in Bartonella genomes. Heme is an iron molecule that carries oxygen. It is part of the “hemoglobin” that makes oxygenated blood red and carries oxygen around the body. Bartonella species cannot make heme on their own, so they must acquire it from a mammal or a vector.
But there’s a problem. Environments that are rich in heme also typically create oxidative stress for the cells. Oxidative stress from toxic by-products of the host’s metabolism can kill bacteria that are not equipped to deal with it.
In a human, Bartonella species get heme from human cells and of course red blood cells are a great source. In the gut of a flea, Bartonella is in a soup of heme. It must be able to deal with both environments. Consequently, instead of having just one method for bringing in heme, a Bartonella species has four or five methods. What if the wrong method is turned on at the wrong time? Or what if the Bartonella species is missing one or two of these genes? The Bartonella cell could be starved or poisoned.
Conclusion
This is just one weakness created by having to live in multiple environments. There are many more, and researchers have found some of them. Knowing more about where a Bartonella species originated and what hosts it passed through as it became a zoonotic pathogen may help point the way to some of these weaknesses.
References
Liu, M. et al. (2012). Heme binding proteins of Bartonella henselae are required when undergoing oxidative stress during cell and flea invasion. PLOS One, 7(10), e48408. https://doi.org/10.1371/journal.pone.0048408 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0048408
Morick, D. et al. (2013). Effects of Bartonella spp. on flea feeding and reproductive performance. Applied and Environmental Microbiology, 79, 3438-3443. doi:10.1128/AEM.00442-13 https://aem.asm.org/content/79/11/3438
Wagner, A., & Dehio, C. (2019). Role of distinct type-IV-secretion systems and secreted effector sets in host adaptation by pathogenic Bartonella species. Cell Microbiology, 21(3), e13004. doi:10.1111/cmi.13004 https://www.ncbi.nlm.nih.gov/pubmed/30644157
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Banerjee, R. et al. (2019). Gene duplication and deletion, not horizontal transfer, drove intra-species mosaicism of Bartonella henselae [Epub ahead of print]. Genomics. doi:10.1016/j.ygeno.2019.03.009 https://www.ncbi.nlm.nih.gov/pubmed/30902757
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