Penicillin G and Cloxacillin in Costa Rican Dairy Products: Quantitative Analysis and Lactic Acid Bacteria Resistance Profiling

<b>Background/Objectives:</b> Milk and dairy products are among the most relevant foods both nutritionally and commercially. Costa Rica stands out as one of the main producers and consumers of dairy products in Central America. However, in recent years, the use of antibiotics in the livestock industry has increased, with implications for public health and food security, generating a need to monitor residues of these drugs in food. The present research focuses on developing a liquid chromatography method for the simultaneous quantification of penicillin G (PEN) and cloxacillin (CLO) in raw and commercial bovine milk, as well as in various dairy products, including fresh cheese and liquid yogurt. <b>Methods/Results</b>: During the validation of the methodology, average sensitivities of (960 ± 8)·10¹ mg L^-1 and (1580 ± 9)·10¹ mg L^-1 were achieved for PEN and CLO, respectively. Determination coefficients of 0.9995 and 0.9998 were also achieved, respectively. The limits of detection (LOD) and quantification (LOQ) for PEN and CLO were (0.330 ± 0.025) mg L^-1 and (0.65 ± 0.12) mg L^-1, (1.10 ± 0.083) mg L^-1 and (2.2 ± 0.4) mg L^-1, respectively, on both accounts. Recoveries were 68-77%, 92-106%, and 78-112% for PEN and 57-79%, 99-114%, and 95-120% for CLO in milk, cheese, and yogurt, respectively, across all three concentration levels evaluated. The precision of the method, in terms of reproducibility, was assessed for liquid yogurt (3-12% RSD_R for PEN and 4-12% RSD_R for CLO) and in cheese (8-14% RSD_R for PEN and 4-12% RSD_R for CLO). Nineteen bovine milk samples from the Cartago area were evaluated, including commercial and milk samples. Additionally, cheese (<i>n</i> = 17) and yogurt samples (<i>n</i> = 8) were analyzed. <b>Conclusions</b>: None of the samples showed detectable signals of the antibiotics. In addition, antibiotic sensitivity testing was performed on <i>n</i> = 9 Lactic Acid Bacteria (LAB) strains isolated from the raw milk samples, revealing the presence of some resistant traits to several antibiotics, including β-lactams.