Trends of Antibiotic Resistance in Mesophilic and Psychrotrophic Bacterial Populations during Cold Storage of Raw Milk

Abstract

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 C or 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 C, often equalled the initial levels and equalled or increased during the cold storage at C, depending on the AB. The study performed at 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 CFU/mL, at levels at which the milk is acceptable for industrial dairy industrial processes.

N2 Gas Flushing Limits the Rise of Antibiotic-Resistant Bacteria in Bovine Raw Milk during Cold Storage

“…Moreover, the highest AR increase for psychrotrophs compared to mesophiles as noted here for samples S2 and S3 (Figures 1, 4) is also in line with previous observations which showed that AR, that was evaluated from psychrotrophs, supplanted AR from the corresponding mesophiles in cold-stored raw milk (Munsch-Alatossava et al, 2012a); this point also highlights the difficulty of judging the quality of raw milk with the sole mesophilic bacteriological standard, in which psychrotrophs may be partially overlooked due to their growth optima as not all psychrotrophs grow under mesophilic conditions.…”

“…Further investigations revealed that psychrotrophic bacterial populations exhibited higher AR levels compared to their corresponding mesophilic populations; most importantly, based on their AR profiles, distinct bacterial populations succeeded one another during cold storage; strangely enough, the AR was highest at the time when the “total bacterial counts” had reached approximately 10 5 cfu/ml; after that, the relative AR level was lower but the milk was spoiled (Munsch-Alatossava et al, 2012a). …”

Characterization of bacterial community of raw milk from dairy cows during subacute ruminal acidosis challenge by high-throughput sequencing

“…To ensure the quality of raw milk, cold storage after milk collection and during transportation is vital. Psychrotrophic bacteria in raw milk are well known for their spoilage potential, and they cause significant economic losses in the dairy industry . Previous studies have shown that Pseudomonas spp.…”

“…Psychrotrophic bacteria in raw milk are well known for their spoilage potential, and they cause significant economic losses in the dairy industry. 40 Previous studies have shown that Pseudomonas spp. are responsible for the spoilage of milk and dairy products, owing to their ability to produce heat-resistant proteolytic and lipolytic enzymes at chilled temperatures.…”

Quality and Safety of Bovine Raw Milk: Present Challenges and Technological Solutions

“…The temperature value, even at low temperature range, is of crucial importance: an illustration can be seen with the 12 raw milk samples considered in experiments I and II, listed in Table 1 [30]; for I and II, the initial average counts in log-units were 3.9 and 4.03, respectively; it can be observed that 4 days cold storage at 4 and 6°C, respectively, yielded bacterial counts of 6.4 and 7.8 log-units, respectively; the 2° shift showed an about 1.4 log-units (equivalent to a factor of 25) higher bacterial level at 6 compared to 4°C. A 2 log-units (a factor of 100) difference in psychrotrophic Pseudomonas levels was also reported by another study that compared optimal (4°C) and suboptimal (6°C) storage temperatures [31].…”