Abstract

Introduction: Aloe vera (Aloe barbadensis Miller) is a medicinal plant that has been used empirically for thousands of years. Among its biological activities, Aloe vera's antimicrobial capacity makes it a possible alternative in combating the evolution of bacterial resistance. The objective of this study was to evaluate the appearance of chromosomal resistance in gram-positive bacteria causing bovine mastitis when exposed to Aloe vera, used either individually or in combination with beta-lactam antibiotics. Methods: The Minimum Inhibitory Concentration (MIC) of Aloe vera, both alone and in combination with cloxacillin or ceftiofur, versus Staphylococcus aureus and Streptococcus uberis was determined by means of the broth macrodilution technique. Serial multipassage techniques allowed us to assess the development of resistance against the described treatments in these strains. Results: After 15 serial passages of Staphylococcus aureus ATCC 29213 (methicillin-sensitive S. aureus; MSSA) and Staphylococcus aureus ATCC 43300 (methicillin-resistant S. aureus; MRSA) versus Aloe vera alone and in combination with cloxacillin or ceftiofur, we did not observe an increase in MIC. However, in serial passages 6 and 14, the MIC for MSSA increased 5.3 and 3.3 fold versus cloxacillin and ceftiofur, respectively, and for MRSA in serial passages 6 and 7 the MIC increased 26.6 and 46.6 fold versus the indicated antibiotics. S. uberis ATCC 29,213 did not register MIC increases after 15 serial passages against Aloe vera alone or in combination with ceftiofur or cloxacillin. Conclusion: The combination of Aloe vera and ceftiofur or cloxacillin delays the appearance of chromosomal resistance in S. aureus strains. Likewise, the synergistic and additive effect of these antibiotics when combined with Aloe vera, would allow for a dose reduction in antibiotics when treating clinical mastitis symptoms of dairy cattle.