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Effect of Ampicillin, Streptomycin, Penicillin and Tetracycline on Metal Resistant and Non-Resistant S. aureus

The seriousness of the problem of bacterial resistance confirms the number of deaths associated with drug-resistant bacterial infections as only in the EU it affects 25,000 people a year [1]. Recently, with the discovery of multi-resistant strains in the broader community, public health officials have begun to concern about the potential danger of the spread of these antibiotic resistant bacteria [2]. Antimicrobial resistance (AMR) is resistance of a microorganism to an antimicrobial medicine, to which it was originally sensitive. The resistance is dependent on the genes located in plasmids that are transferred to other cells, thus the resistance between bacteria spreads rapidly. Resistant microorganisms are able to withstand an attack of antimicrobial medicines, so that standard treatments become ineffective and infections persists increasing risk of spreading to others. The evolution of resistant strains is a natural phenomenon that happens when microorganisms are exposed to antimicrobial drugs and resistant strains can be exchanged between certain types of bacteria. The misuse of antimicrobial medicines accelerates this phenomenon.

The incidence of resistant microorganisms is monitored primarily in hospitals, but higher risk of resistant bacterial strains occurrence can be found in the soil and waters [3]. Resistance generating in the external environment is a natural development of every live system. Infections caused by resistant microorganisms often fail to respond to the standard treatment, resulting in prolonged illness and greater risk of death [4,5,6,7]. The death rate for patients with serious infections treated in hospitals is about twice that in patients with infections caused by non-resistant bacteria [8]. A high percentage of hospital-acquired infections are caused by highly resistant bacteria such as methicillin-resistant S. aureus (MRSA) or multidrug-resistant enterococci Gram-negative bacteria. The general mechanisms of resistance are: the limited penetration of antibiotics into the bacterial cells, the change of the target structure (receptor), metabolic changes within the bacterial cell, which prevents the effect of antibiotics on the target structures, enzymatic inhibition/inactivation of antibiotics [9,10,11,12,13,14]. Metal resistance of microbes is accomplished by intra- and extracellular mechanisms. Metals can be excreted via efflux transport systems, sequestering compounds of the cytosol can bind and detoxify metals inside the cell. The release of chelators into the extracellular milieu leads to bound and fixed metals. The structure of the cell envelope is prone to bind large amounts of metals by sorption thus preventing influx [15]. New discovered resistance mechanisms, such as enzymes produced by the bacteria that destroy last generation antibiotics, have emerged among several Gram-negative bacilli and have rapidly spread to many countries. This can render ineffective powerful antibiotics, which are often the last defence against multi-resistant strains of bacteria. This new resistant mechanism is encountered in ordinary human pathogens (e.g. Escherichia coli) that causes common infections such as urinary tract infection.

The use of antibiotics and toxic metals should be considered carefully with the attention paid to the environmental impacts [16,17]. Heavy metals are toxic and can be harmful to organisms. For this reason, a number of organisms including bacteria develop processes which are able to withstand the effects of these pollutants [15]. Toxic metals in the environment can select for and enrich bacteria with antibiotic multiresistance [18]. Resende et al. [19] evaluated medically relevant bacteria in an aquaculture system and their susceptibility to antibiotics and toxic metals. Multidrug-resistant bacteria were also tolerant to nickel, zinc, chromium and copper. In another study by Ji et al. [20] eight antibiotic resistance genes (ARG), seven heavy metals and six antibiotics were quantified in manures and soils collected from multiple feedlots in Shanghai. Overall, sulfonamide ARGs were more abundant than tetracycline ARGs. The significant positive correlations were found between some ARGs and typical heavy metals such as Cu, Zn and Hg. Similarly, in the study by Malik et al. [21] majority of the Pseudomonas isolates from water and soil exhibited resistance to multiple metals (Hg, Cd, Pb, Cu, Zn, Ni) and antibiotics (water - tetracycline, polymyxin B; soil – sulphadiazine, ampicillin and erythromycin) was presented [21]. With the appearance of antibiotic-resistant bacteria, the increasing numbers of infections cause huge losses to both economic concerns and social resources over recent decades, and this has become a global problem [2].

This study aimed on studying of behaviour of non-resistant strain of bacterial culture S. aureus and S. aureus strains resistant to heavy metals ions (cadmium = RCd or lead = RPb) exposed to different concentrations of four kinds of antibiotics (ampicillin, streptomycin, penicillin and tetracycline). The antimicrobial activity of antibiotics on S. aureus bacterial culture was tested by the growth curves and the results were statistically evaluated.

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