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What are Bacteria?

Bacteria are single celled organisms that are too small to see without a microscope. Unlike higher animals, they do not have a nucleus in their cells. They are one of the simplest forms of life but are still incredibly numerous and successful. It is estimated that up to 50 million bacteria may be living in a single gram of garden soil while a milliliter of fresh water may hold a million bacteria.
Bacteria are classified by their basic forms:

    • cocci – Under the microscope, these bacteria are circular in shape. Their scientific names names end in coccus. For example, the bacteria that cause strep throat are Streptococcus bacteria. Cocci can occur in clumps or in chains.



    • bacilli These bacteria are rod-shaped. A common bacteria in our gut, E. coli, is an example of a rod shaped species.



  • spirochaetes A third group of bacteria are spiral in shape. Syphilis, yaws and Lyme’s disease are all caused by spirochaetes.



    Bacteria also come in two basic forms based on a staining test. Gram positive bacteria take up the crystal violet stain used in the Gram staining method of bacterial differentiation while gram negative bacterial species do not. Streptococcus and Staphylococcus bacteria that cause Legionnaire’s disease and salmonella food poisoning are both gram positive. The bacterial species that causes Gonorrhea, Neisseria gonorrhoeae, is gram negative.
    Another way to differentiate bacteria is by whether they need oxygen (aerobic bacteria) or do not need oxygen (anaerobic bacteria). Some anaerobes are beneficial to humans, but others can cause illnesses, such as appendicitis, diverticulitis, and gingivitis. Characteristics of an anaerobic bacterial infection are bad-smelling pus, the formation of abscesses, and the destruction of tissue. Aerobic bacteria can also be either harmful, like Staphyloccus, or helpful, such as the bacteria that break down sewage in waste water treatment systems.
    There are thousands of species of bacteria that we come into contact with in our lives. Most are harmless, some are beneficial and, luckily, only a few cause disease. Those that do are called pathogens. A pathogen is any organism that can cause a disease.

  • Pathogenicity is a measure of the ability of an organism to cause disease.
  • Virulence is the measurement of how deadly that organism is to its host. The genetics and structure of the bacteria and its biochemical composition determine whether it will have beneficial or destructive effects on its host species.

The Host – Pathogen Relationship

    The relationship between a bacterial species and its host is fluid and changing. Bacteria have short lifespans and produce large numbers of offspring so they can adapt quickly to changing conditions. They can evolve to adapt to new hosts or to become resistant to treatments and medicines such as antibiotics.
    Hosts change more slowly because they have longer generation times and fewer offspring. However, they can develop resistance over time and evolve new defense mechanisms to fight infections. Many pathogens that were deadly when they first appeared in human populations are no longer as virulent as they once were. This has occurred as the human population exposed to the pathogen has changed. The weaker and more vulnerable victims die. Those that survive pass on their resistance to their offspring.
    Bacteria are infamous for their ability to develop resistance to antibiotics. New strains of syphilis, gonorrhea and Staphyloccus bacteria have developed that are not affected by penicillin and other commonly used antibiotics. This is a dangerous example of evolution in action. It is a biological arms race between pathogens and their hosts that is constantly changing. It tests our ability to adapt and survive as a species and as individuals.

Mechanisms of pathogenicity:

  • Invasiveness is the capability to infect a host. A pathogen can be deadly but if it cannot infect us then we are safe. Some disease organisms have terrible effects on their hosts but they are not very invasive. The bacteria that cause Hansen’s Disease, more commonly known as leprosy, is like this. Sufferers were locked away in leprosariums because of the fear of this terrible condition where limbs would rot and fall off. However doctors and nurses could work for years among the patients and never become infected because of the low level of invasiveness of the bacteria that causes it, Mycobacterium leprae.
  • Toxogenicity is the ability to produce toxins. Some bacteria are highly invasive. They are often in the soil and our environment and can quickly enter a wound and cause infection, but they are not terribly toxic. That is, they do not produce the killer toxins that would make them deadly to us. On the other hand, the tetanus bacteria, Clostridium tetani, produces a toxin that affects the brain and nervous system. The powerful toxin, tetanospasmin, impairs the victim’s motor neurons and nerves that control their muscles, causing the locked jaw effect which gives the disease its common name.


    Bacteria live in soils and waters. Everything we touch has bacteria on it. Most are not inclined to infect us but a few are actively seeking a host. These are the pathogenic bacteria.



    Our bodies are built to fight bacteria and other small invaders. Our largest organ is our skin, which is a tough wrapping that is designed to resist infectious, pathogenic organisms like bacteria. However there are bacteria that are adapted to live on our skin. They can tolerate the dry salty conditions and live on dead skin cells in return. They are part of a normal healthy body flora and fauna from the mites in our eyebrows to the bacteria in our guts and on our skin. Many are harmless, some are very useful, even necessary, for our survival and a few are pathogens, ready to harm us for their own purposes of survival and reproduction.
    Most of these pesky and sometimes dangerous species enter via our orifices, like the mouth, nose, eyes and genital openings or via cuts and abrasions to our skin. Bacteria adapted for living in warm, wet environments often cannot survive away from a body for very long. These species need direct contact to transfer from one host to another, so kissing and sexual acts are perfect for spreading bacteria from one body to a new host.
    Finding new hosts is the most dangerous and yet most vital part of the pathogenic bacterial life cycle. This is why evolution favors parasites that do not kill their hosts. The longer the host lives, the more opportunities are available for spreading to new host individuals. There is no advantage to the parasite to kill its host before transmission to new hosts has been successful. This is why over time many bacterial diseases have become less virulent and deadly.

Methods of transmission:

  • through the water: like hepatitis, dangerous, because the parasites are outside the body for long periods and can be killed when ingested by healthy gut conditions and flora.
  • through the air: like cold ‘germs’ (sometimes bacterial, sometimes viral), usually by being sneezed out into the air and then breathed in by the next potential victim. also dangerous for the bacteria since if they do not get ingested, they die.
  • through contamination of food: fecal coliforms are spherical bacteria like E coli that live in our guts. E coli is used as an indicator species for fecal contamination of food and drink. Many pathogenic bacteria travel from one host to another via fecal contamination and it is a relatively safe way to move to a new host. Basically the bacteria leave one body via feces which are then transmitted to another host or food or water through bad hygiene practices such as not washing one’s hands after having a bowel movement. Bacteria transmitted in this way cause a number of food poisoning diseases including listeria, salmonella and botulism.
  • Direct transmission from host to host: When we hug and kiss, bacteria and other organisms that depend on us for survival use these opportunities to infect a new host. Sexual activity is even better for many pathogens because they are going from a warm, moist and favorable habitat to a new warm, moist and favorable habitat with little or no exposure to the harshness of the cold world outside a mammalian body. Unfortunately for free love advocates, promiscuous sexual behavior can lead to the acquisition of dangerous freeloaders capable of transmitting such diseases as syphilis and gonorrhea, among others, to one’s partners. The best way to avoid sexually transmitted diseases is to practice celibacy or stick with one uninfected partner. Safe sex practices also help but are not fool proof. If mistakes are made, direct transmission occurs and people’s lives change.
  • Through an intermediate host: Some bacteria and other pathogens have evolved a complicated strategy to move from host to host and this involves an intermediate host. These are called vectors. Rats and lice are common vector species for some nasty human pathogens including the bacteria that caused the Black Death and still strikes down people in some places even now (although it does seem to be less virulent now than it was in the Middle Ages). Lice suck the blood of infected rats and then suck human blood. The lice die but the pathogen has made the journey from the rat to a human host.


    Once a pathogenic bacteria enters a new host, millions of years of evolution kick into action as coded instructions in the bacterial DNA are acted upon.  The bacteria journey to the organs which they evolved to infect. Once in the most favorable environment, they multiply. In the process, they produce chemicals that are toxins to the host, causing the development of disease symptoms.
    Our bodies do what they can to fight the disease. They produce antibodies. They send white blood cells to kill the invaders. Many of the disease symptoms are actually caused by the host’s body as it fights the disease. Fever occurs as the body raises its temperature to kill the invaders. Pus is made up of the dead bodies of both bacteria and white blood cells. If the host body is healthy with a strong immune system, the invader can be defeated. However, in many cases it has already been passed to a new host and in this sense, the bacterial disease has won. It does not matter that those left behind are doomed to die. If the infection has spread to new hosts, then the disease organism has been successful.

This short video shows how bacteria cause disease.


  • Todar’s Online Textbook of Bacteriology

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