Overview of Bacteria

ByBrian J. Werth, PharmD, University of Washington School of Pharmacy
Reviewed/Revised May 2024
View Patient Education

    Bacteria are microorganisms that have circular double-stranded DNA (except for Streptomyces species) and cell walls (except for mycoplasma species). Most bacteria live extracellularly, but some preferentially reside and replicate intracellularly.

    Obligate intracellular pathogens are able to grow, reproduce, and cause disease only within the cells of the host. Examples of these pathogens include chlamydiae and rickettsiae.

    Facultative intracellular pathogens are able to live and reproduce either inside or outside of host cells. Examples of these pathogens include Salmonella typhi, Brucella species, Francisella tularensis, Neisseria gonorrhoeae, N. meningitidis, Legionella and Listeria species, and Mycobacterium tuberculosis.

    Many bacteria are present in humans as normal microbiota, often in large numbers and in many areas (eg, in the gastrointestinal tract and skin). Only a few bacterial species are human pathogens.

    Bacteria are classified by the following criteria (see also table Classification of Common Pathogenic Bacteria):

    • Morphology

    • Staining

    • Encapsulation

    • Oxygen requirements

    Morphology

    Bacteria may be

    • Cylindric (bacilli)

    • Spherical (cocci)

    • Spiral (spirochetes)

    A few coccal, many bacillary, and most spirochetal species are motile.

    Staining

    Gram stain is the most common stain for general bacterial identification. Gram-positive bacteria retain crystal violet dye (appearing dark blue) after iodine fixation, alcohol decolorization, and counterstaining with safranin; gram-negative bacteria, which do not retain crystal violet, appear red. Gram-negative bacteria have an additional outer membrane containing lipopolysaccharide (endotoxin), increasing the virulence and antibiotic resistance of these bacteria. (For other factors that enhance bacterial pathogenicity, see Factors Facilitating Microbial Invasion.)

    and Kinyoun stain are acid-fast stains used to identify mainly mycobacteria, particularly M. tuberculosis. They also can identify the gram-positive bacilli Nocardia and the protozoans Cryptosporidia, Cyclospora, and Cystoisospora

    Fluorochrome stains (eg, auramine-rhodamine) also identify acid-fast organisms, but a special fluorescent microscope is required.

    (See also Microscopy.)

    Encapsulation

    Some bacteria are enclosed in capsules; for some encapsulated bacteria (eg, Streptococcus pneumoniae, Haemophilus influenzae), the capsule helps protect them from ingestion by phagocytes. Encapsulation increases bacterial virulence.

    Oxygen requirements

    Aerobic bacteria (obligate aerobes) require oxygen to produce energy and to grow in culture. They produce energy using aerobic cellular respiration.

    Anaerobic bacteria (obligate anaerobes) do not require oxygen and do not grow in culture if air is present. They produce energy using fermentation or anaerobic respiration. Anaerobic bacteria are common in the gastrointestinal tract, vagina, dental crevices, and chronic wounds when blood supply is impaired.

    Facultative bacteria can grow with or without oxygen. They produce energy by fermentation or anaerobic respiration when oxygen is absent and by aerobic cellular respiration when oxygen is present. Microaerophilic bacteria prefer a reduced oxygen tension (eg, 2 to 10%).

    Microaerophilic bacteria can grow in environments where oxygen is limited. Examples include Campylobacter species and Helicobacter pylori.

    Obligate intracellular parasites, such as chlamydiae, acquire energy from the host cell and do not produce it themselves.

    Table
    Table
    Drugs Mentioned In This Article
    quizzes_lightbulb_red
    Test your KnowledgeTake a Quiz!
    Download the free Merck Manual App iOS ANDROID
    Download the free Merck Manual App iOS ANDROID
    Download the free Merck Manual App iOS ANDROID