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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.

Colonies of Salmonella typhimurium growing on an agar plate. Courtesy of Dr. M. Pränting.

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.

Gram-positive bacteria are surrounded by a plasma membrane and a thick mesh-like peptidoglycan cell wall (left). Gram-negative bacteria are surrounded by an inner membrane, a thin peptidoglycan layer and an outer membrane.

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:

Gram-positive bacteria

Infection types (examples)

Streptococcus pneumoniae (pneumococci) Pneumonia, meningitis
Staphylococcus aureus (S. aureus) Wound infections
Streptococci Throat and wound infections
Clostridioides difficile (C. difficile) Enterocolitis


Gram-negative bacteria

Infection types (examples)

Escherichia coli (E. coli) Urinary tract infections
Klebsiella pneumoniae Urinary tract infections
Salmonella, Campylobacter Gastroenteritis
Neisseria meningitidis (meningococci) Meningitis
Neisseria gonorrhoeae (gonococci) Gonorrhea
Haemophilus influenzae Pneumonia, meningitis

Find out more

There are many different ways to learn online. Below you’ll find two examples of free online platforms where you can learn more about microbes in the human body and their importance for us.

e-Bug is an educational resource to play games and learn about microbes, hygiene, infection, and prudent antibiotic use aiming at junior and senior school children. The website is currently available in a variety of different languages and is developed in collaboration with teachers and stakeholders from partner countries across Europe (led by Public Health England).

Your Microbial Friends is an interactive tool to explore the normal flora and is developed by the Genetic Science Learning Center (University of Utah). Please visit their site and find two worksheets designed to help you to organize and take notes on the content covered in this online resource.

© Uppsala University