Bacterial cells lacking flagella, rendering them non‑motile.
Explanation
Atrichous bacteria are defined by the absence of flagella—whip‑like appendages used for motility in many prokaryotes. While monotrichous, lophotrichous, amphitrichous and peritrichous bacteria bear one or more flagella arranged in distinct patterns, atrichous cells have none and therefore cannot swim actively. Lack of flagella does not preclude other surface structures; atrichous bacteria may possess pili, fimbriae or capsules that mediate adherence, conjugation or gliding motility. Most atrichous species are transported passively by fluid flow or host movements, and their ecological niches often do not require active motility. In laboratory identification, observing the absence of flagella is part of bacterial classification and can aid in differentiating genera.
Examples and significance
Many clinically and industrially important bacteria are atrichous. Gram‑positive examples include Staphylococcus aureus and other staphylococci, Streptococcus pyogenes and other streptococci, and Bacillus anthracis. Gram‑negative atrichous organisms include Klebsiella pneumoniae, Shigella dysenteriae and Pasteurella multocida. Lactic acid bacteria such as Lactobacillus acidophilus and Lactococcus lactis are also non‑flagellated and play roles in fermentation and as probiotics. The absence of flagella influences virulence: Shigella species invade the intestinal epithelium by actin polymerization rather than swimming, while Bacillus anthracis uses its protective capsule and toxins for pathogenesis. Some atrichous bacteria can exhibit surface‑associated gliding or twitching motility driven by pili, but they lack the rotational flagellar motor system.
Classifying bacteria by flagellar arrangement helps microbiologists interpret motility tests and anticipate ecological strategies. Atrichous bacteria demonstrate that active swimming is not essential for survival and that other mechanisms can enable pathogenicity and adaptation.
Related Terms: Monotrichous, Lophotrichous, Peritrichous, Flagellum, Pili