There are only a few reports describing the isolation of antibiotic-resistant C. trachomatis
strains from patients [50
]. Although 11 of the 15 reportedly resistant isolates were associated with clinical treatment failure, all of the isolates screened displayed characteristics of ‘heterotypic resistance’, a form of phenotypic resistance in which a small proportion of an infecting microbial species is capable of expressing resistance at any one time. This phenomenon has also been described in Staphylococcus
], and parallel observations of similar phenotypic resistant states can be referred to in the literature as drug indifference, persistence, tolerance and, in some cases, as properties of biofilms [58
]. It is possible that these descriptors of bacterial interactions with antibiotics can be associated with chlamydial aberrancy and phenotypic antibiotic resistance in Chlamydiae. For example, tolerance is often specific to antibiotics that affect cell wall synthesis, as is shown in the penicillin persistence model of Chlamydiae [58
In each case of clinical resistance reported, only a small portion of the population (<1–10%) expressed resistance, and those that did also displayed altered inclusion morphology. In addition, the isolates could not survive long-term passage (in the presence or absence of antibiotics) or lost their resistance upon passage. In some cases, heterotypic resistance was observed when a large inoculum was infected on to cells, but a smaller inoculum was not resistant under the same conditions [50
]. Many of these characteristics suggest that a form of phenotypic resistance is responsible for the sustained presence of small populations of clinical strains of C. trachomatis
under antibiotic stress and may be an adaptive behavior that influences the survival of bacteria within communities rather than stable genetic resistance mechanisms employed by singular cells.
A distinct characteristic of chlamydial growth is the asynchronous differentiation of RBs to EBs that begins relatively early and continues throughout the developmental cycle. A midstage inclusion will harbor actively dividing RBs as well as nondividing EBs. It is plausible that multistage development is an evolved trait that can ensure the survival of a subset of the population regardless of the timing of antibiotic or metabolic stress. AZM, clarithromycin, levofloxacin and ofloxacin approach 100% inhibition in synchronized assays, but when used in a continuous model of C. pneumoniae
infection, none of these antibiotics eliminated the organism, even in the presence of concentrations greater than four-times their minimum inhibitory concentrations (MICs) [39
]. A continuous model may more accurately reflect in vivo
infections because inclusions of varying developmental stages will be present at any given time. The standard MIC assay synchronizes the infection and applies antibiotics within 1–2 h post infection, long before EB differentiation can be observed. Perhaps chlamydia are most vulnerable in the log-phase of growth prior to EB differentiation, and are capable of expressing phenotypic resistance when both replicating and nonreplicating forms are present. This principle is corroborated by other studies, in particular one in which ciprofloxacin and ofloxacin failed to eradicate C. trachomatis
in infected cells and induced persistence when antibiotics were applied to established infections (2–3 days post infection) [41
]. Although it is assumed that the inclusion is a nutrient-rich environment, it is unknown whether adequate nutrient levels can support replication and sustain active metabolism, or whether toxic byproducts accumulate, particularly in the late stages of the developmental cycle when several hundred bacteria occupy a single inclusion. These factors may also contribute to the onset of phenotypic or heterotypic resistance observed both in vivo
and in vitro
It is challenging to distinguish persistence from issues of treatment compliance, re-infection of treated patients and actual antibiotic resistance in Chlamydiae. It remains even more challenging to assess the relevance of heterotypic resistance when it is observed in strains isolated from patients with clinical treatment failure. In the absence of true genetic differences, it is challenging to find a way to study antibiotic resistance that arises only under certain conditions in approximately 1% of the population and which often does not appear to manifest itself following expansion of the bacteria.