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    • The reasons why antibiotic treatment

    2023-09-28

    The reasons why antibiotic treatment may be detrimental are diverse. First, it may result in disruption of multifaceted microbe–microbe, as well as microbe–host, interactions that are key for the maintenance of homeostasis within this ecosystem. This can lead to the expansion of pathogenic microorganisms, otherwise suppressed by the presence of surrounding commensal populations that may compete for similar nutrient sources (a phenomenon known as ‘colonization resistance’). Also, it may promote the expansion of antibiotic-resistant strains and/or lead to the acquisition of antibiotic resistance by otherwise susceptible bacteria, including pathogens (Box 3). These mechanisms can contribute to irreversible alterations of the microbiota composition [2,3], potentially leading to loss of beneficial and/or over-representation of deleterious species. Finally, reduced stimulation of the immune system by commensal bacteria could undermine the appropriate ‘education’ of the immune system, leading to the development of immune responses against self or innocuous antigens and predisposing individuals to autoimmunity or allergy.
    Antibiotic Use and Epidemiology of Allergic Diseases Various factors have been proposed to explain the rapid increase in the incidence of allergy in industrialized countries over the past century. These include reduced contact with diverse environmental microorganisms [34,35] or change in dietary habits [36–39]. Regarding the first hypothesis, there are two main facets of exposure to environmental microorganisms: (i) contact with diverse nonpathogenic microbes that do not drive disease or clinical symptoms but modulate the immune system (e.g., through induction of Tregs) and, thus, potentially protect against development of allergy later in life; and (ii) contact with pathogens that cause host infections and drive host inflammatory responses (e.g., Th1/17) that result in clinical symptoms, but at the same time, potentially protect from developing Th2-related diseases (e.g., allergies) later in life. One of the first reports pointing to the positive correlation between the use of 5823 synthesis and allergy was published in 1998. The authors found a positive association between antibiotic use during the first 2 years of life and occurrence of atopic disorders [40]. This observation was supported by a study published the following year, showing a marked association between antibiotic use and asthma in children. In this work, the odds ratio was higher when antibiotics were used during the first year of life than when used after the first birthday (Table 1). Also, a higher frequency of antibiotic administration resulted in a higher odds ratio for asthma development [41]. Similar conclusions were reached a year later by Droste and colleagues, who showed a positive correlation between antibiotic use during the first year of life and atopy. Importantly, after data stratification for the existence of parental hay fever, this association was only significant for children whose parents were atopic [42]. These data implicated a possible role for crosstalk between genetic predisposition and environmental insult in the manifestation of allergy. These are three out of at least six studies [40–45] performed before 2002 that all pointed to the positive correlation between antibiotic use and atopy. Importantly, they were all retrospective studies (i.e., relied on questionnaires). This approach may not be ideal since it could introduce ‘recall bias’ to the study, especially since parents with atopic children may visit medical doctors more frequently and be more likely to report the use of antibiotics. Additionally, a positive correlation between antibiotic use and allergy may be confounded by reverse causation (i.e., when the use of antibiotics is merely the consequence of having allergy, which is misdiagnosed as an infection, due, for example, to similar symptoms, such as cough). To minimize these issues, prospective birth cohort studies were launched in which children’s medical histories were followed from birth to a certain age. From 2001 to 2004, at least four such studies were performed [46–49], one of which supported [47] and three refuted [46,48,49] a positive association between antibiotic use and atopy. The first meta-analysis of published reports conducted in 2006, taking into account both retrospective and prospective studies, pointed towards an overall positive correlation between antibiotic use and atopy, although the effect in prospective studies was modest [50].