A Yao | PubMed Health

PubMed2026

Genomic and food-safety evaluation of Staphylococcus chromogenes in Chinese dairy milk.

Non-aureus staphylococci and mammaliicocci (NASM) cause mastitis and may contaminate milk and dairy products. Milk samples (n = 1916) from cows with subclinical or clinical mastitis (SCM and CM, respectively) were collected from 28 large-scale (> 500 lactating cows) Chinese dairy farms. Overall, 999 NASM isolates representing 19 species were identified by MALDI-TOF MS and cpn60 sequencing, with Staphylococcuschromogenes, Mammaliicoccus sciuri and Staphylococcus haemolyticus being most prevalent. Antimicrobial resistance (AMR) was determined with disc diffusion; non-susceptible to penicillin was most common (SCM, 30% and CM, 29%) whereas cefoxitin non-susceptible NASM accounted for 8-10% of isolates; among these, 12.5% carried mecA but none carried mecC. Galleria mellonella was used to assess virulence of 78 strains of S. chromogenes, a dominant species; subsequently, 32 strains, representing higher- and lower-virulence in the Galleria model, were selected for whole-genome sequencing and comparative genomics. S. chromogenes isolates from CM had higher virulence (p < 0.05) than those from SCM. The 32 genomes comprised 20 sequence types, indicating high genetic diversity. No robust genomic marker of Galleria virulence phenotype was identified in this selected WGS subset. Acquired resistance genes (n = 5) were detected, including a first report of fusC in S. chromogenes; the fusC-positive isolate had an elevated fusidic acid MIC (8 mg/L). Although S. chromogenes persisted in milk at 4 °C, pasteurization (64 °C for 30 min) reduced viable counts to below detection. This study provided new insights into the prevalence, AMR, genomic diversity, and dairy-chain relevance of milk-derived NASM, particularly S. chromogenes. However, the genomic findings were based on an intentionally selected WGS subset and should be interpreted as hypothesis-generating rather than population-representative.

Animals↗

PubMed1992

Sensitivity of transformation to small differences in population density during serial passage of NIH 3T3 cells.

Early passages of the NIH 3T3 mouse cell line undergo spontaneous neoplastic transformation leading to the development of transformed foci if grown to confluence in 2% (vol/vol) calf serum (CS) and left there for more than a week. Transfer of the postconfluent cultures results in the appearance of large numbers of transformed foci; many of them are larger and denser than those in the original culture. If the cells are continually kept at low population densities by frequent passages in 10% CS, they lose the capacity to undergo spontaneous transformation. If however the low-density passages are made in 2% CS or in 10% (vol/vol) fetal bovine serum, both of which support lower growth rates and saturation densities than does 10% CS, they gain the capacities to grow to high saturation densities and produce more foci when grown to confluence in 2% CS. These increases are proportional to the population densities used in the frequent passages, although the densities are all kept well below confluence. We conclude that the combined constraints of submaximal serum plus those of the limited cell contacts of the low cell densities used here elicit an adaptive response that endows the entire population with increased growth capacity. The increased growth capacity of the heterogeneous population in turn increases the capacity of a fraction of the population to initiate distinctive transformed foci. Similar studies have indicated that the capacity of cells to produce tumors and metastases in mice and rats is enhanced by prior maintenance at high density in culture. We propose the concept of progressive state selection to account for the general increase in the growth capacity of cells that is elicited by moderate constraints on their growth and metabolism.

3T3 Cells↗