Psychrotrophic bacteria in raw milk are most well known for their spoilage potential and cause significant economic losses in the dairy industry. Despite their ability to produce several exoenzyme types at low temperatures, psychrotrophs that dominate the microflora at the time of spoilage are generally considered benign bacteria. It was recently reported that raw milk-spoiling Gram-negative-psychrotrophs frequently carried antibiotic resistance (AR) features. The present study evaluated AR to four antibiotics (ABs) (gentamicin, ceftazidime, levofloxacin, and trimethoprim-sulfamethoxazole) in mesophilic and psychrotrophic bacterial populations recovered from 18 raw milk samples, after four days storage at 4 ° C or 6 ° C. Robust analysis of variance and non parametric statistics (e.g., REGW and NPS) revealed that AR prevalence among psychrotrophs, for milk samples stored at 4 ° C, often equalled the initial levels and equalled or increased during the cold storage at 6 ° C, depending on the AB. The study performed at 4 ° C with an intermediate sampling point at day 2 suggested that (1) different psychrotrophic communities with varying AR levels dominate over time and (2) that AR (determined from relative amounts) was most prevalent, transiently, after 2-day storage in psychrotrophic or mesophilic populations, most importantly at a stage where total counts were below or around 1 0 5 ？CFU/mL, at levels at which the milk is acceptable for industrial dairy industrial processes. 1. Introduction In developed countries, the sanitation of raw milk is monitored by “total” bacterial counts or SPC (standard plate count). The standard for Grade A or 1 raw milk is an SPC value less than 1.0 × 105？CFU/mL [1]. Cooling of raw milk to less than 6°C (typically 3 to 4°C in the farm tank following milking), followed by storage at temperatures below 6°C during transportation to the dairy plant, ensures the quality of raw milk. Because milk is a highly suitable growth medium, many bacterial genera have adapted to this cold-temperature environment by producing exoenzymes (such as proteases or lipases) that can withstand the typical heat treatments milk is subjected to, and they consequently cause significant economic loss to the dairy industry [2–5]. Psychrotrophic bacteria (able to grow at temperatures ranging from 0°C to 7°C) [6] are well known for their ability to degrade both raw and processed milk components which may explain why raw milk psychrotrophs are mainly considered for their spoilage features, with some exceptions like the human pathogens Bacillus cereus
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