Simple Summary
On 21 April 2025, size-resolved airborne bacterial samples were collected from 10 family-backyard small-scale beef cattle farms in Wuzhong, Ningxia, using a six-stage Andersen sampler, followed by culture recovery and 16S rRNA V3–V4 sequencing. Coarse fractions (upper airway) showed higher diversity than fine (alveolar) fractions; community differentiation among the six fractions was statistically significant but weak (ANOSIM, R = 0.0443, p = 0.003). Bacillota dominated (~85%) and was enriched in alveolar fractions, while Bacillus was the most abundant genus and enriched in the finest fraction; pathogen-containing genera (Bacillus, Acinetobacter, Corynebacterium, Staphylococcus) showed size-dependent distributions, suggesting potential respiratory exposure risk. Given the culture-dependent cross-sectional design and lack of species-level resolution, actual risk awaits further verification. The study provides baseline size-resolved data for farm biosafety management.
Abstract
Ningxia has a large number of small-scale beef cattle farms, particularly those using the family backyard rearing system. However, the diversity and distribution of airborne bacteria in these farms remain poorly understood. In this study, airborne microorganisms were collected from small-scale beef cattle farms using an Andersen-6 air microbial sampler (BY-300, Jiangsu, China). Airborne particles were separated into six size fractions according to the sampling stages. The composition and diversity of bacterial communities in different fractions were analyzed using 16S rRNA gene high-throughput sequencing. The Chao and Shannon α-diversity indices were significantly higher in the coarse-particle fraction than in the alveolar fraction. Bacterial community structure differed significantly among the six size fractions (ANOSIM, R = 0.0443, p = 0.003, 999 permutations), although the overall differentiation was weak. Only 5.95% of bacterial taxa were shared among all fractions. The microbial communities consisted of a small proportion of persistent core taxa and a large proportion of transient taxa. The proportion of persistent and intermittent taxa increased with decreasing particle size. The mean relative abundance of bacterial taxa was positively correlated with their detection frequency (R2 = 0.351, p < 0.000001). At the phylum level, Bacillota was the predominant phylum. Its relative abundance increased with decreasing particle size and was significantly enriched in the alveolar fraction. The abundances of“Pseudomonadota and Actinomycetota” differed significantly among size fractions. At the genus level, Bacillus had the highest relative abundance and was enriched in the ultrafine alveolar fraction. Acinetobacter, Corynebacterium, and Staphylococcus showed distinct size-fraction distributions. Overall, airborne bacterial communities in small-scale beef cattle farms showed statistically significant but weak differentiation among size fractions. Different size fractions may be associated with different levels of respiratory exposure. We detected genera with known pathogenic species, such as Bacillus, Acinetobacter, Corynebacterium and Staphylococcus, in aerosol samples. The V3–V4 16S rRNA sequencing could not reliably identify these bacteria to species level. The presence of these genera in the alveolar fraction indicates a possible risk of respiratory exposure, which needs further study. These findings provide basic data on size-fraction-resolved airborne bacterial communities in small-scale beef cattle farms in Ningxia and may support further research on biosafety control in small-scale beef cattle production.