The Antibacterial and Antibiofilm Activities of Silver Nanoparticles on Staphylococci Isolates From Cow Milk

Abstract

Biofilm-producing ability has been identified as a serious virulence factor in staphylococci and increases their antimicrobial resistance. This study aimed to investigate the biofilm forming ability of staphylococci isolated from cow milk samples. Moreover, we assessed the antibiofilm activity of silver nanoparticles (AgNPs) against methicillin resistant (MRSA) and biofilm forming staphylococci. The results revealed that 82.14% (23/28) and 91.66% (11/12) of the coagulase positive staphylococci (CPS) and coagulase negative Staphylococci (CNS) isolates, respectively produced biofilm on Congo red agar (CRA). In the case of the microtiter plates (MTP) method, all CPS and CNS isolates produced biofilm at different levels. These results revealed a substantial agreement between CRA and MTP results according to the kappa coefficient test (kappa value = 0.773). Staphylococcus aureus species specific nuc gene and the determinant of methicillin resistance (mecA) gene were amplified from S. aureus isolates (n=10). The intercellular adhesion gene A (icaA), and intercellular adhesion gene D (icaD) were amplified from all the CPS and CNS isolates, but none of the isolates were positive for the biofilm associated protein (bap) gene. Antibiotic susceptibility testing showed that all isolates were oxacillin resistant. AgNPs (100 μg/mL) inhibited the growth of Staphylococci isolates (inhibition zone diameters ranged from 22 to 28 mm). AgNPs decreased biofilm formation in the biofilm forming Staphylococci isolates with percent ranged from 67.05 to 98.02% using MTP assay. In conclusion, AgNPs have antistaphylococcal activity and inhibit biofilm formation.