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HIV is one of many diseases that can influence the composition of the. However, gut flora can vary significantly from person to person depending on certain behaviors. To explore how sex practices and substance use can alter the organisms in the intestines, a team of researchers at UCLA and Cedars-Sinai Medical Center in Los Angelesarchived specimens and clinical data on a group of 37 relatively young HIV-positive men who have sex with men.The subjects, who were part of a larger study on drug abuse that took place from 2014 to 2016, had given rectal sponge specimens, blood and urine samples, and answers to detailed questions about sexual practices and drug use at roughly 6-month intervals for the duration of the study. The samples yielded DNA, which was then sequenced to provide profiles of individual microbiomes.The scientists found that the men who said they used substances had microbiomes that were different from healthy controls who were studied for comparison. Specifically, they were likely to have an increased number of Fusobacterium and Anaerotruncus and a decreased number of Dorea organisms if they used marijuana. Those who used methamphetamines also had higher levels of Fusobacterium and Anaerotruncus, along with Granulicatella, but lower levels of Parabacteroides, Collinsella, and Paraprevotella, among others.Sexual behaviors changed their gut makeup as well. Participants who reported performing oral sex (with ejaculation) showed higher levels of Granulicatella and Clostridium cluster XIVa organisms but lower levels of Campylobacter, Actinomyces, Anaerococcus, Finegoldia, and Ruminococcus. Being the receptive partner in anal intercourse was shown to increase Anaerococcus and Peptostreptococcus but lower Clostridium complex IV, Dorea, and Anaerotruncus, as well as others. How recently and how frequently a subject engaged in these behaviors also seemed to have an effect on the makeup of the microbiome.Why is an altered microbiome significant? “The intestinal microbiome is essential for digestive functions such as nutrient absorption as well as local immune functions,” Jennifer A. Fulcher, MD, PhD, assistant professor-in-residence at UCLA and an author of the study, told Contagion. “Alterations in the microbiome have been associated with diseases such as autoimmune disease, inflammatory bowel disease, and many infections such as HIV. Some bacteria thrive in an inflammatory environment and can further increase inflammation, such as Prevotella. Increases in bacteria such as these are thought to increase systemic inflammation, which can over the long term increase the risk of inflammation-related diseases such as cardiovascular disease.”However, microbiome changes may not necessarily be negative in every person. “Certainly, many of the bacteria within the microbiome are beneficial, and we are still working to understand the health effects of much of the microbiota,” Dr Fulcher said. “In many studies, maintenance of a Bacteroides-dominant microbiome has been associated with reduced health risk.”The study was limited by its small size and by its exclusion of several possible confounding variables such as diet and the use of lubricant, certain types of which can cause epithelial damage during intercourse that may lead to a change in the microbiome. The team is working on putting together larger studies that will confirm their most recent findings, as well as looking at how the bacteria identified in this study affect local mucosal immune cells. “Other studies examining the effects of restoring the microbiome on these health outcomes will be important in the future,” Dr Fulcher said.Ms. Saloman is a health writer with more than 20 years of experience working for both consumer-and physician-focused publications. She is a graduate of Brandeis University and the Medill School of Journalism at Northwestern University. She lives in New Jersey with her family.