Mollicutes are a distinct group of bacteria, recognized for their exceptionally small size and unique biological characteristics. They are among the smallest self-replicating organisms, offering insights into the fundamental requirements for cellular life. Their study has implications across various fields, including veterinary science, agriculture, and human health.
Unique Biological Features
Mollicutes differ from most other bacteria due to their complete absence of a rigid cell wall. This means they lack the peptidoglycan layer that provides structural integrity to nearly all other bacterial cells. Without a cell wall, Mollicutes exhibit a highly flexible and variable shape, known as pleomorphism. Their pliable outer membrane allows them to squeeze through filters and intercellular spaces that would typically impede other bacteria.
These organisms are also remarkably small, typically measuring 0.2 to 0.3 micrometers in diameter. They have significantly reduced and simplified genomes compared to most other bacteria. For instance, Mycoplasma genitalium has one of the smallest known bacterial genomes. This genomic reduction reflects their parasitic or commensal lifestyle, as they rely on their hosts for essential nutrients like amino acids, nucleotides, and lipids.
Role as Pathogens
While some Mollicutes are harmless, many species are significant pathogens across various hosts. In humans, Mycoplasma pneumoniae commonly causes respiratory infections, often called “walking pneumonia.” This bacterium attaches to respiratory tract cells, leading to symptoms like persistent cough, fever, and fatigue.
Mollicutes also threaten animal health, impacting livestock and companion animals. Various Mycoplasma species cause respiratory diseases, arthritis, and mastitis in cattle, sheep, and goats, leading to economic losses. In poultry, Mycoplasma gallisepticum causes chronic respiratory disease in chickens and turkeys.
Beyond animals, phytoplasmas and spiroplasmas are notable plant pathogens causing agricultural damage. Phytoplasmas cause numerous plant diseases with symptoms like yellowing, stunted growth, and abnormal flower development. Spiroplasmas, recognized by their helical shape, also infect plants, leading to conditions like citrus stubborn disease. Both are transmitted between plants by insect vectors.
Detection and Treatment Difficulties
The unique biological features of Mollicutes present considerable challenges for their detection and treatment. Their complete lack of a cell wall renders them naturally resistant to a broad class of commonly used antibiotics. Antibiotics like penicillin and other beta-lactams work by targeting bacterial cell wall synthesis. Since Mollicutes do not possess this structure, these drugs are ineffective.
Mollicutes infections are typically treated with antibiotics that act on other bacterial components, such as macrolides, tetracyclines, or fluoroquinolones. However, widespread use has led to increasing resistant strains, complicating treatment strategies and underscoring the need for careful antibiotic stewardship.
Furthermore, their extremely small size and fastidious growth requirements make them difficult to detect and culture in laboratory settings. Many species require specific, nutrient-rich media, often supplemented with cholesterol, and can take an extended period to grow, if they grow at all. Phytoplasmas, for instance, are obligate parasites and have not yet been successfully cultured on artificial media. This slow and demanding cultivation process complicates diagnosis. Nucleic acid amplification tests, such as PCR assays, offer significantly higher sensitivity for detection, providing a more reliable diagnostic alternative to traditional culture methods.