Background Bacterial infections account for a large proportion of global morbidity and mortality. While antimicrobial drugs have helped save millions from the consequences of these infections, emerging antimicrobial resistance now threatens to reverse those gains. The HIV-epidemic renders large populations drastically more susceptible to bacterial infections, particularly in Sub-Saharan Africa, where HIV is rampant and health-systems to deal with the consequences are insufficient. Indiscriminate use of antimicrobials is thought to be the main driving force behind emerging resistance. The opportunistic infection Pneumocystis jirovecii pneumonia (PJP) is a major cause of mortality among HIV-infected people with low immunity. Continuous cotrimoxazole preventive therapy (CPT) has proved to reduce morbidity and mortality from PJP globally, as well as in Africa where the burden of HIV is largest. The impact of this large-scale antibiotic use on emergence of antimicrobial resistance in Africa remains unclear. In a recent literature review, Sibanda and colleagues found only two studies specifically designed to answer the question of whether CPT induced resistance in common bacterial pathogens, and 15 other studies that assessed the question as sub-analyses of studies designed for other purposes. The two targeted studies both assessed resistance in pneumococci, one finding increased resistance to clindamycin in the CPT group, neither finding any difference in resistance to penicillin. While most of the other 15 studies assessed penicillin-resistance in pneumococci and methicillin-resistance in Staphylococcus aureus (MRSA), only two studies evaluated the effect on Gram-negative bacteria. Among these, the study from San Francisco showed significantly higher temporal increase from 1988 to 1995 in resistance to cotrimoxazole, ampicillin, cefazolin and gentamicin in clinical isolates of E. coli from HIV-positives than from HIV-negatives. The study of HIV-infected children with pneumonia in Cape Town did not find any association between CPT and resistance in clinical isolates, including 26 isolates of Klebsiella pneumonia, the most prevalent Gram-negative microbe in this study. In previous research in Tanzania, we found that multi-drug resistance in Gram-negative bacteria is prevalent and a problem of major public health concern, as invasive infections with such bacteria was associated with a large increase in mortality. Our study found case-fatality rates (\>70%) approaching those of the pre-antibiotic era in systemic bacterial infections with multi-resistant Gram-negative bacteria harboring extended-spectrum beta-lactamase (ESBL) resistance. ESBLs are enzymes that render the all-important antibiotics, penicillins and cephalosporins, useless. We also confirmed previous findings that case-fatality rates from bacterial sepsis was higher in HIV-infected than HIV-negative patients. Most disturbingly, our study confirmed that in multidrug-resistant Gram-negative bacteria the gene encoding for ESBL resistance was transferred by plasmids together with resistance to several other antibiotics, including cotrimoxazole. Thus, it is plausible that widespread cotrimoxazole use could lead to selection of multidrug-resistant Gram-negative bacteria in the gut of HIV-patients, which consequently could contribute to the rapid spread of these harmful and difficult-to-treat bacteria. In high-resource countries, it has been shown that CPT can safely be discontinued when CD4 counts increase beyond 200/uL. Recent studies from Sub-Saharan Africa indicate that prolonged CPT after immune restoration to CD4 counts \> 200/µL by antiretroviral treatment (ART) may be beneficial as it is associated with reduced hospitalization from malaria and bacterial infections in children. There was, however, no difference in mortality among those who stopped and continued CPT. In the "2014 supplement to the WHO guidelines on the use of antiretroviral drugs for treating and preventing HIV infection", the recommendation for CPT use is widely expanded for settings with high prevalence of malaria and bacterial infections, where it is recommended that all patients with HIV start CPT regardless of CD4 counts and clinical stage and receive prolonged CPT for indefinitely time. Tanzanian national guidelines recommend discontinuing CPT when patients are stabilized with CD4 counts \>350/µL, which is a more extensive recommendation than European guidelines, but much less extensive than the WHO recommendations for "settings with high prevalence of malaria and bacterial infections".

Cotrimoxazole has been widely used for a number of different indications and, as a result, cotrimoxazole resistance in general has increased. Given the limited published data on these important public health issues, we designed this study to address the potential impact of prolonged CPT on antimicrobial resistance development with particular focus on Gram-negative gut microbes, VRE (vancomycin-resistant enterococci), and nasal carriage of MRSA. If the study should confirm a marked increase in carriage of multidrug-resistant bacteria in patients on prolonged CPT this needs to be considered when developing new guidelines. We also aim at assessing whether restoration of immunity by ART could be a way to counteract this threat of antimicrobial resistance. This proposed randomized clinical trial will be performed in the setting of the ongoing Dar es Salaam - Bergen collaborative initiative and will include local investigators and laboratory workers, along with Norwegian investigators with scientific experience from clinical and molecular studies in Tanzania.

Problem statement/ research gap CPT has been standard of care for preventing PJP and other infections among HIV-infected patients with CD4 counts \< 200µL since the early 1990s. The recent recommendation by WHO to give CPT indefinitely to HIV-infected people in resource-constrained settings, even to those with initial good immunity (CD4 counts above 350/µL) and to those whose immunity has been restored by antiretroviral treatment, would lead to a massive increase in cotrimoxazole consumption in Sub-Saharan Africa, which has the largest burden of HIV-infection. While this recommendation is based on limited scientific data regarding short-term morbidity, there is no evidence of improved survival and the potential harmful implication for selection of multidrug-resistant bacteria in HIV-patients on prolonged CPT has not been comprehensively studied.

Rationale of the study Continuing CPT indefinitely stable good immunity with CD4 counts \>350/µL has been shown to reduce morbidity, but not mortality, in persons living with HIV in resource-constrained settings. Complying with recommendation for continuation of CPT indefinitely in patients with restored immunity and virological control would imply a massively increased consumption of cotrimoxazole in HIV-endemic countries. The risk of increased carriage of multidrug-resistant bacteria among HIV-patients on prolonged CPT has not been comprehensively studied, but has potentially grave implications for public health, since systemic infections with these resistant bacteria are very difficult to treat and associated with very high mortality rates. The current study is designed to assess whether prolonged CPT is associated with increased carriage of multidrug-resistant bacteria. The results of this study may have important implications for development of new guidelines on CPT use for resource-constrained settings and may help preventing emerging antibacterial resistance.