The rod-shaped tuberculosis (TB) bacterium, which the World Health Organization has once again ranked as the top infectious disease killer globally, is the first single-celled organism ever observed to maintain a consistent growth rate throughout its life cycle. These findings, reported by Tufts University School of Medicine researchers on November 15 in the journal Nature Microbiology, overturn core beliefs of bacterial cell biology and hint at why the deadly pathogen so readily outmaneuvers our immune system and antibiotics.

"The most basic thing you can study in bacteria is how they grow and divide, yet our study reveals that the TB pathogen is playing by a completely different set of rules compared to easier-to-study model organisms,” said Bree Aldridge, a professor of molecular biology and microbiology at the School of Medicine and a professor of biomedical engineering at the School of Engineering, as well as one of the paper’s co-senior authors along with Ariel Amir of the Weizmann Institute of Science.

TB bacteria are successful at surviving in humans because some parts of the infection
can quickly evolve within their host, allowing these outliers to avoid detection or resist treatment. If someone has TB, it takes months of various antibiotics to be cured, and even then, this approach is only successful in 85 percent of patients. Aldridge and her colleagues hypothesize that gaps in our understanding of the basic biology behind this phenomenon have been holding back the development of more effective treatments.

Getting answers, however, proved to be slow and meticulous work. Postdoctoral fellow Christin (Eun Seon) Chung at the School of Medicine, one of the paper’s first authors, spent three years in a specialized facility equipped to handle high-risk pathogens observing the behavior of individual TB cells. Because TB bacteria double every ~24-hours (compared to 20 minutes for several model bacterial species), Aldridge’s team needed to develop and deploy new microscopy methods to film the microbe over week-long periods. Chung analyzed the footage and tracked each TB bacterium and their progeny manually as they are also notoriously small and prone to move about, so automated analysis could not be used.

These experiments showed that the TB bacterium doesn’t follow expected patterns of cell growth. In other bacterial species, growth is exponential, which means cells grow slower when they are smaller. For TB bacteria, growth rates can be the same whether they are newly born (and small) or far along in their cell cycle and soon to divide.

“This is the first reported organism that can do this,” said Chung. “TB’s behavior challenges fundamental bacterial biology as it’s been thought that ribosomes—which are sites of protein synthesis in the cell—drive cell growth rates, but our work suggests that something else may be happening in TB bacteria that raises new questions about its growth control.”

In addition to reporting that there is extensive variation in growth behaviors among the individual bacterial cells, the team discovered another new growth behavior of TB bacteria: they can also begin growing from either end after being born. This was unexpected as related bacteria only start growing from the end opposite of where they pinched off their mother cell at division.

Together, the observations reveal that TB microbes use alternative strategies to increase variability among their offspring, challenging previous assumptions based on faster-growing and more uniform model organisms. Aldridge says the study will help her lab and other research teams better understand and exploit these mechanisms for treatment purposes.

"A lot of basic microbiology research is done in fast-growing model organisms, and while they're models for a reason, that doesn't make them representatives of other types of bacteria," said Aldridge. "There's an enormous diversity of life that we're not studying at the fundamental level and this work demonstrates why we need to study the pathogens themselves.”

Prathitha Kar of Harvard University, the other co-first author on the paper, and Maliwan Kamkaew, formerly of Tufts University School of Medicine, also contributed to the work.