Superficial and invasive infections caused by
Staphylococcus aureus continue to raise serious health challenges globally. These acute and chronic infections have now become more problematic after emerging multi-drug resistance (MDR) against various frontline antibiotics [
1]. The problem of MDR in
S. aureus is subsequently emerging both in nosocomial and hospital-acquired settings, with a significantly higher mortality and morbidity rate [
2,
3]. The condition of drug resistance is primarily developed by unregulated sales of antibiotics, a long course of medication, indiscriminate usage of drugs and poor public health infrastructure. Statistically, incidences of methicillin-sensitive
S. aureus (MSSA), methicillin-resistant
S. aureus (MRSA) and vancomycin-resistant
S. aureus (VRSA) infections have been endemic as it steadily increased up to 54% [
4‐
6]. According to Indian hospitals survey, over 80% clinical samples of
S. aureus were established resistance to the frontline antibiotics including methicillin [
7]. However, the prevalence of MSRA infection in the U.S. accounts for 94,000 cases and over ~ 18,000 deaths per year [
8]. Despite it, readily biofilm formation on the medical devices and host tissues also contribute to the persistent chronic infections. Biofilm embedded
S. aureus remarkably decreased antibiotic and immune-defence susceptibility by over 100 folds and making them difficult to treat clinically [
9,
10]. The process of biofilm formation is multifactorial among which polysaccharide intercellular antigen (PIA) synthesized from UDP-N-acetylglucosamine via intercellular adhesion (
icaADBC) locus play an important role. It has been reported that point mutant in the
icaADBC locus abrogated the capacity of biofilm formation in
S. aureus [
11]. In contrast, secretions of virulence factors during different growth phase also contribute to biofilm formation [
12]. Combating this notorious infection remains a major challenge as most of the conventional antibiotics have now become redundant to work and an immediate treatment regimen required for its elimination. Much efforts have gone into devising a workable treatment against staphylococcal infections particularly for the elimination of MRSA, VRSA pathogens by 1) searching a new antimicrobial from different sources 2) repurposing a new therapeutic property of the known drugs 3) synergizing the efficacy of the antibiotics with combination of others. Identification of the new antimicrobials via de novo synthesis and screening are a slow, costly and traditional approach. Most of the on-going research work to identify a new antimicrobial is focused on to repurposing existing drugs with known therapeutic property and toxicity that remarkably reduce treatment cost and side-effects with an antibiotic development. Simvastatin which originally used for the treatment of cardiovascular disease as it decreases the cholesterol level has also showed antimicrobial property against list of Gram-positive pathogens [
13]. Similarly, anti-inflammatory, anti-oxidant ebsleen, antineoplastic 5-fluoro-2′-deoxyuridine (FdUrd) and anti-rheumatoid auranofin have also been reported to possess a strong bactericidal effect on drug-resistant; MRSA and VRSA strains [
13,
14]. Combining two or more therapeutic agents is another lucrative approach for synergizing treatment and promptly elimination of pathogens by reducing the antibiotics load of the individual drug. Interactions between different antibiotics pair were found be synergistic against MSSA, MRSA, and Pseudomonas acquired infections. Plectasin paired either with β-lactam or aminoglycoside
, Vancomycin increased potency of gentamycin, Nordihydroguaiaretic acid enhanced antimicrobial activity of aminoglycosides, glycerol monolaurate
(GML) and lauric acid either with streptomycin or gentamicin are such examples [
15‐
17]. In addition, the synergistic interaction of antibiotic with a variety of different compounds has also been reported. For example, hydroisothiocynates synergistically inhibited the growth of
S. aureus with streptomycin. The polymyxin B boosted doxycycline and trgecycline activity against doxycycline-resistant and susceptible
K. pneumoniae clinical isolates [
18].
Ouabain or g-strophanthin is a cardiotonic steroid [
19], derived from the plants (
Strophanthus gratus, Acokanthera schimperi) or secreted endogenously by the adrenal glands [
20]. It acts on α-subunit of Na
+/K
+-ATPase to inhibit its transport activity resulting in an increase intracellular sodium ion (Na
+) concentration [
21,
22]. The physio-pathological role of ouabain is linked to increase the heart contraction in patients with congestive heart failure [
23]. However, disruption of Na
+K
+-ATPase activity leads to disturb membrane polarity with subsequent accumulation of intracellular Ca
2+ level and neurotransmitter release [
24]. A group of studies has demonstrated that ouabain also promotes different types of cell proliferation [
25‐
29] and cell susceptibility against different microbes [
30]. However, the efficacy of ouabain on persistent
S. aureus biofilm and functional mechanism for the treatment of Staphylococcal infection has not elucidated so far. With this background, the aim of the present study is to evaluate in vitro activity of ouabain together with aminoglycosides (GEN, KAN, AMK) and other antibiotics (AMP, TET, VAN) against
S. aureus.