Bacteria basics

Bacteria are found all around us; in the air we breathe, in the soil and water and inside and on our bodies.

Bacteria are tiny single-celled organisms, only a few micrometers in size, and the individual cells can only be seen under a microscope. On some surfaces, for example when grown on agar plates in the laboratory, bacteria can form colonies that can be made up of several hundred thousands of cells and thus are visible to the eye.

Gram-positive and Gram-negative bacteria and common pathogens

Bacteria can be broadly categorized into two groups, Gram-positive and Gram-negative bacteria. The groups are determined based on staining and microscopy, in which some bacteria will turn purple (Gram-positive bacteria) and other pink/red (Gram-negative bacteria). This is in turn related to the structure and composition of the cell wall surrounding the bacterial cell. The structure of the cell wall is also one of the factors that influence bacterial susceptibility to antibiotics.

_{Cell envelope structure of Gram-positive and Gram-negative bacteria. Peptidoglycan: mesh-like cell wall layer; cytoplasm: area inside the cell membrane; LPS: lipopolysaccharide, molecule found in the outer membrane of Gram-negative bacteria; Porin: pore in the membrane; OM: Outer membrane; IM: inner membrane; Periplasmic space: area between OM and IM. Courtesy of Dr. E. Gullberg.}

Some of the most common Gram-positive and Gram-negative bacteria, and infections they give rise to, are summarized in the tables below:

© Uppsala University