Detection of antibacterial-like activity on a silica surface: fluoroquinolones and their environmental metabolites

Abstract

Background, scope, and aims Antibacterial fluoroquinolones (FQs) are third-generation antibiotics that are commonly used as therapeutic treatments of respiratory and urinary tract infections. They are used far less in intensively farmed animal production systems, though their use may be permitted in the veterinary treatments of flocks or in medicated feeds. When used, only a fraction of ingested parent FQ actually reaches the in vivo target site of infection, while the remainder is excreted as the parent FQ and its metabolized products. In many species’ metabolism, enrofloxacin (EF) is converted into ciprofloxacin (CF) while both FQs are classified as parent FQs in human treatments. It is therefore likely that both FQs and their metabolic products will contribute to a common pool of metabolites in biological wastes. Wastes from intensive farming practices are either directly applied to agricultural land without treatment or may be temporarily stored prior to disposal. However, human waste is treated in sewage treatment plants (STPs) where it is converted into biosolids. In the storage or treatment process of STPs, FQs and their in vivo metabolites are further converted into other environmental metabolites (FQEMs) by ex vivo physicochemical processes that act and interact to produce complex mixtures of FQEMs, some of which have antibacterial-like activities. Biosolids are then often applied to agricultural land as a fertilizer amendment where FQs and FQEMs can be further converted into additional FQEMs by soil processes. It is therefore likely that FQ-contaminated biowaste-treated soils will contain complex mixtures of FQEMs, some of which may have antibacterial-like activities that may be expressed on bacteria endemic to the receiving agricultural soil environment. Concern has arisen in the scientific and in the general community that repeated use of FQ-contaminated biowaste as fertilizer amendments of nutrient-impoverished agricultural land may create a selective environment in which FQ-resistant bacteria might grow. The likelihood of this happening will depend, to some extent, on whether bioactive FQEMs are first synthesized from the parent FQs by the action and interaction of in vivo and ex vivo processes producing bioactive FQEMs in biowastes and biosolids. The postulated creation of a selective environment will also depend, in part, on whether such bioactive FQEMs are biologically available to bacteria, which may, in turn, be influenced by soil type, amendment regime, and the persistence of the bioactive FQEMs. Additionally, soil bacteria and soil processes may be affected in different ways or extents by bioactive FQEMs that could possibly act additively or synergistically at ecological targets in these non-target bacteria. This is an important consideration, since, while parent FQs have well-defined ecological targets (DNA gyrase and topoisomerase IV) and modes of bactericidal action, the FQEMs and their possible modes of action on the many different species of soil bacteria is less well studied. It is therefore understandable that there is a lack of conclusive evidence directly attributing biosolid usage to any increase in FQ-resistant bacteria detected in biowaste-amended agricultural soil. However, a lack of evidence may simply imply that a causal relationship between biosolid usage programs and any detection of low levels of FQ-resistant bacteria in soils has yet to be established, rather than an assumption of no relationship whatsoever. Based on results presented in this paper, the precautionary principle should be applied in the usage of FQ-contaminated biosolids as fertilizer amendments of agricultural land. The aim of this research was to test whether any bioactive FQEMs of EF could be synthesized by aerobic fermentation processes using Mycobacterium gilvum (American Tissue Culture Collection) and a mixed culture of microorganisms derived from an agricultural soil. High-performance thin-layer chromatography (HPTLC) and bioautography were tested as screening techniques in the detection and analysis of bioactive FQEMs.