In Vitro Susceptibility of Pythium insidiosum Isolates to Aminoglycoside Antibiotics and Tigecycline

Abstract

bThis study evaluated the in vitro activity of aminoglycoside antibiotics and tigecycline against Pythium insidiosum. The susceptibility tests were carried out using the broth microdilution method in accordance with the CLSI document M38-A2. MIC values for gentamicin, neomycin, paromomycin, and streptomycin ranged from 32 to 64 mg/liter, and the minimal fungicidal concentration (MFC) ranged from 32 to 128 mg/liter, which are incompatible with safe concentrations of these drugs in plasma in vivo. Tigecycline showed the lowest MIC (0.25 to 2 mg/liter) and MFC (1 to 8 mg/liter) range values. The in vitro susceptibility observed to tigecycline makes this drug a good option in future tests in vitro and in vivo for the management of pythiosis. Pythiosis is a chronic disease that affects humans, mammals, and birds and is caused by the aquatic oomycete Pythium insidiosum. It occurs in tropical, subtropical, and temperate regions, and there have been no reports of direct transmission among animals or between animals and humans. Cutaneous, vascular, ocular, and systemic forms (humans) and subcutaneous and gastrointestinal forms (horses, dogs, cattle, cats, and sheep) are the clinical presentations most commonly observed (3).Pythiosis progresses rapidly and can become life threatening if it is not treated in the early stages. The absence of ergosterol in the cell wall of this oomycete restrains the treatment of pythiosis with antifungal therapy due to the fact that most antifungal drugs act by inhibiting the synthesis of ergosterol. Radical surgery and immunotherapy are among the best therapeutic options, but the improved cure rates are directly associated with early diagnosis of pythiosis (14).Members of the genus Pythium are known to be susceptible to some antibiotics of the tetracycline, macrolide, aminoglycoside, and amphenicol classes (5,6,8,12). McMeekin and Mendoza (9) reported that streptomycin may have a stimulatory or inhibitory effect on P. insidiosum depending on the culture conditions, and Loreto et al. (7) described an in vitro inhibitory activity of minocycline against P. insidiosum. In this context, this study aims to evaluate the in vitro susceptibility of the aminoglycosides gentamicin, neomycin, paromomycin, and streptomycin and the minocycline-derived tigecycline against isolates of P. insidiosum.Twenty-three clinical Brazilian isolates of P. insidiosum from equine pythiosis cases and the ATCC 58.637 reference strain were tested in this study. The identities of the isolates were confirmed by a PCR-based assay (4).Gentamicin, neomycin, paromomycin, and streptomycin were obtained from Sigma Chemical Co. (St. Louis, MO). Tigecycline (Tygacil; Pfizer) was purchased commercially. The drugs were dissolved in dimethyl sulfoxide to obtain stock solutions (12,800 mg/ liter) and stored at Ϫ70°C.At present, there is no standardized susceptibility testing method for oomycetes. Because P. insidiosum produces hyphae and based on the results of previous in vitro and in vivo studies (7,11), the susceptibility test wa...

In Vitro and In Vivo Antimicrobial Activities of Minocycline in Combination with Azithromycin, Clarithromycin, or Tigecycline against Pythium insidiosum

“…The in vitro susceptibility results in our study were consistent with those of previous studies (7)(8)(9)(10). In vivo, the previous studies of induced pythiosis in rabbits demonstrated that caspofungin (1 mg/kg/day) (14), some combinations of terbinafine (125 mg/day) with itraconazole (5 mg/kg/day), fluvastatin (1 mg/kg/day), or caspofungin (1 mg/kg/day) (15), and a vortexed immunotherapy (30 mg at 14-day intervals) (12, 16) presented fungistatic activity, i.e., the progression of subcutaneous lesions was less than that for the untreated group for some therapies, but complete remission of the lesions was not the prevalent observation.…”

“…Despite a few reports of successful therapy with antifungal drugs (2,3), many studies have demonstrated their ineffectiveness (1,(4)(5)(6). We previously described antibacterial compounds belonging to the glycylcycline, macrolide, and tetracycline classes (7)(8)(9)(10) as having potent in vitro activity against P. insidiosum. The present study investigated the in vitro and in vivo interactions among minocycline, azithromycin, clarithromycin, and tigecycline against P. insidiosum.…”

In Vitro Susceptibility of Brazilian Pythium insidiosum Isolates to Essential Oils of Some Lamiaceae Family Species

“…Antifungal-based therapies have proved to be difficult due to the absence of ergosterol in the cytoplasmic membrane of P. insidiosum, in view of the fact that ergosterol is the action site of the most commonly used antifungals available [2]. The lack of effective therapeutic alternatives for pythiosis has increased the interest in investigating the activity of antifungal [4,6] and non-antifungal agents [5,7] and their associations with other agents [8].…”

In Vitro Synergism Observed with Azithromycin, Clarithromycin, Minocycline, or Tigecycline in Association with Antifungal Agents against Pythium insidiosum

“…Previous studies have shown that the growth of P. insidiosum is inhibited in vitro by the glycylcycline, macrolide, and tetracycline classes of antibacterial drugs (5,6). However, studies evaluating the antimicrobial combination of antibacterial and antifungal agents against P. insidiosum have not been performed.…”