Bacteria are the oldest species known on earth and are first seen in the fossil record approximately 3.5 billion years ago.


Bacteria are highly diverse and adaptable, having been found in every type of habitat on earth from the icy waters of Antarctica to the hot thermal springs in California.

Most bacterial species are environmental, only a relatively small proportion are human pathogens.

Pathogenic species (such as Legionellae) have evolved to grow preferentially at human body temperatures, therefore they can be killed at relatively low temperatures.  Most skin and oral bacteria are killed between 60°C to 65°C which is useful as this means that tunics and uniforms worn in the dental surgery can be disinfected in domestic washing machines at these temperatures.

Mycobacteria Tuberculosis (the cause of Tuberculosis), has a special bacterial wall containing mycolic acids and thick walled spore forming bacteria such as Clostridium species which causes tetanus, botulism, gas gangrene, C. difficile and antibiotic-associated colitis. This is heat resistant and requires higher temperatures such as 135°C to 137°C generated under pressure in a steam steriliser to destroy them.

Bacteria are characterised by their oxygen requirements for growth:

Aerobic Bacteria

Require oxygen for their respiration and growth

Anaerobic Bacteria

Cannot grow in the presence of oxygen

Facultative Anaerobes 

Prefer growth in the presence of oxygen, but can survive and grow in the absence of oxygen

Most dental infections are in the main, but not exclusively, polymicrobial.

Typically, an acute dental abscess comprises of facultative anaerobes (such as viridans group streptococci and the Streptococcus anginosus group), plus predominantly strict anaerobes (such as anaerobic cocci, Prevotella and Fusobacterium species).

The vast majority of dental abscesses respond to surgical treatment such as drainage of pus and elimination of the source of infection with root treatment. Antibiotic use is limited to spreading infections or patients with systemic illness, such as a raised temperature.

In other polymicrobial oral infections where anaerobic bacteria predominate (such as periodontitis and pericoronitis) oxygen generating mouth washes containing hydrogen peroxide are used as an adjunct to root planning and curettage to inhibit anaerobes.

Antibiotic Resistance

Bacteria are prokaryotes unlike fungi and protozoa, which are eukaryotes.

Bacteria have structurally different chromosomes to eukaryotes, lack a nuclear membrane and mitochondria and are bounded by a peptidoglycan cell wall.

Bacteria can transmit and take up small pieces of DNA known as plasmids. These carry DNA sequences that code for antibiotic resistance. Plasmids can be transmitted to both the same species and between unrelated species. Antibiotic resistance can spread readily within bacterial biofilms such as dental plaque and can be spread from person to person.

Antibiotic resistance can also arise due to mutations in DNA replication and are selected for by clinical antibiotic use and misuse.  Hence the importance of strict antibiotic stewardship practiced in hospital and dentistry in order to prevent the spread of multidrug resistant bacterial species.