The Most Common Bacteria and Their Effects on Humans

Page content


If you want to understand the many bacteria in the world, it’s helpful to have a system to keep all of them straight. Bacteria are by far the most numerous and diverse life forms on Earth. Originally scientists had only what they could see under the microscope or on a petri dish to guide them in organizing microbes. Since those days, genetic and genomic sciences have given us a clearer picture of just how bacteria are related to one another.

Through a fusion of the old style (based on phenotype) and the new age techniques (based on genotyping), we have the modern systems and nomenclature for organizing bacteria. Earlier generations of scientists were not that far off the mark with the vast majority of bacteria they classified. For that reason, most of the names (often describing bacterial traits) have stuck, even though the odd species has needed to be moved over to a more closely related grouping.

One final thing to understand: most bacteria do not readily cause disease in humans because they simply didn’t evolve the tools to survive in our bodies. Here below we have a few that did…

What is a Pathogen?

Let’s start with a simple definition:

Pathogen - “a disease-causing organism” (Toratora, G.J., Funke, B.R., Case, C.L. Microbiology: an introduction. 8th ed. 2004)

By its very definition a pathogen must actually make you sick. As you probably know there are many many bacteria inside your body. In fact, there are more bacterial cells inside of you than human cells. This is due to the fact that your cells are much larger than bacteria, so bacteria make up in numbers what they lack in volume. All told, your body contains about a pound of bacteria!

So why aren’t you sick all the time? Well, ironically this is due in large part to the number of bacteria that DO live inside you. Bacteria are highly competitive and constantly jostle for position in the gut and other highly colonized areas. Where one bacteria lives, another cannot. Pathogens, while good at causing disease, are typically not very good at interspecies competition and frequently lose out to the natives inside you (called commensals). These organisms cause disease only when something is out of balance and spend the majority of their time helping digest our food, among other things.

Common Pathogenic Bacteria

Harmful bacteria, of course, aren’t the only things out there that cause disease (viruses, prions, parasites, and fungi each do their fair share), but they’re very common and have very complex relationships with your body.

Here are a few of the ones you might hear about regularly.

Normal Flora

These species are normally good neighbors in and on our bodies, but sometimes they get out of control.

  • Escherichia coli - A rod-shaped bacterium that serves an important role in the human digestive system, but can cause gastroenteritis, urinary tract infections (UTI’s), and diarrheal diseases. Closely related to Shigella species.
  • Staphylococcus aureus - Clusters of spherical bacteria that frequently cause skin infections, but can be involved in much more serious infections of the lungs, heart, brain, and other parts of the body.

  • Staphylococcus epidermidis - Clusters of spherical bacteria that typically do not cause diseases except in patients with under-performing immune systems.

  • Streptococcus pneumoniae - Chains of spherical bacteria that comprise the most common form of pneumonia (infection of the lungs), but can also infect other parts of the body.

  • Streptococcus pyogenes - Chains of spherical bacteria that cause common infections like strep throat or more life-threatening problems such as Toxic Shock Syndrome.

  • Pseudomonas aeruginosa - A rod that typically does not cause disease in humans unless given an explicit opportunity (thus and “opportunistic organism”). Responsible for some secondary infections, hospital-borne infections, and Cystic Fibrosis in the lungs.

  • Clostridium botulinum - A rod-shaped bacterium whose toxins are responsible for Botulism, an often lethal paralysis, in the canning industry

  • Clostridium tetani - A rod-shaped bacterium whose toxins are responsible for Tetanus, a spastic paralysis that causes “lockjaw” and sometimes respiratory failure.

  • Clostridium difficile - A rod-shaped bacterium that can become dominant in the gut after a course of strong antibiotics and cause a diarrheal disease.

  • Enterrococcus species - Spherical cells that cause opportunistic infections, especially problematic because of their tendency to be resistant to many antibiotics.

  • Haemophilus influenzae - An opportunistic rod that can cause pneumonia, bacterial meningitis, and other infections. The name stems from an early belief that it caused influenza before the viral nature of the disease was known.

Other Pathogenic Bacteria

Are there other bacteria types out there that cause disease but DON’T live in your body as commensals? Mycobacterium tuberculosis is an example of an organism that lives in many people, but most of them are “carriers.” It isn’t typically considered a commensal, but instead a dormant pathogen.

Other organisms, like Shigella, do not normally inhabit their hosts, but remain in a population by means of a contaminated water system. Shigella follows the “fecal/oral” route and leaves patients with a dysentery known as Shigellosis.

Further Reading

Learn more at the ASM’s MicrobeLibrary, where you can see pictures and videos of bacteria and how microbiologists manipulate them!

Another good place to direct questions is MedlinePlus from the NIH.