How do Viruses and Bacteria Cause Disease?
Published on the 29 September 2016
Published on the 29 September 2016
Bacteria and viruses are microbes (germs) which are very different to each other in structure and function.
Bacteria can grow on cell-free media; they have rigid cell walls that are susceptible to the action of antibiotics and they can be seen (and smelled) growing in colonies on agar plates, or in Gram or other stains when looked at under the microscope.
Viruses, however, are 10 to 100 times smaller than bacteria. They cannot be seen in stains except with an electron microscope and they can only grow in artificial media containing living cells. They have a different structure (morphology) to bacteria and are not susceptible to antibiotics.
In spite of these important structural and cultural differences, both bacteria and viruses can cause disease in similar ways: they invade and multiply within the host by evading the immune system.
Try listing all the viruses you know, the diseases they cause, and their associated symptoms to remind you of their variety and complexity.
Viruses tend to target specific tissues (cells) in the host.
For example, the influenza virus has a predilection for the respiratory tract, hepatitis viruses target the liver, poliovirus targets the motor neurones of the spinal cord and rotavirus multiplies in the gut. Symptoms of a viral infection may be subtle and nonspecific or specific and suggestive of the causative agent.
Dengue virus, Ross river virus, measles and rubella infections are associated with fever and a widespread red rash, chicken pox and herpes simplex viruses are associated with blistering, often localised, rashes; and hepatitis viruses cause liver damage and jaundice.
Bacteria tend to be less tissue-specific and non-discriminatory than viruses and can cause a variety of infections once they have invaded the host.
These bacterial infections are often manifested by the presence of pus wherever the bacteria settle, and systemic symptoms such as fevers, chills, pain, swelling and loss of function occur when bacteria invade and multiply.
For bacteria and viruses to cause disease they must first colonise the patient.
Once they have invaded the host they can multiply readily. If the host is immune to the virus or bacteria due to previous exposure or vaccination, these organisms may not be able to cause disease. The person’s immune system detects their presence and eliminates them.
However, if there is an imbalance in the host (e.g. the person is immune-compromised or non-immune), the microbe is virulent. Small numbers are capable of causing illness, however if there is exposure to large numbers of microbes that can paralyse or overcome the immune system, and the host is a nutritious target, then disease may result.
Bacteria and viruses do not have brains to organise their attack on the host. Their threat lies in their ability to multiply quickly (in around 20 minutes for most bacteria). Moreover, they can rapidly change their molecular structure by mutation or selection to ensure their survival in adverse conditions.
It is vital for healthcare professionals to be especially aware of the dangers of viruses and bacteria to avoid contamination and the risk of spreading infection to vulnerable patients.
We can protect ourselves from many viral and bacterial diseases by using the vaccines that have been developed to boost our immunity so that we are not vulnerable when exposed to these microbes. We can also practice avoidance techniques by using personal protective equipment (PPE) to protect our skin and mucous membranes from contact with these organisms. Thirdly, we should wash our hands regularly to ensure that any microbes that we have picked up inadvertently from the environment do not get a chance to be inhaled, inoculated or ingested.
I challenge you to incorporate these strategic actions into your practice and help others to understand and adopt similar safe living strategies.
Compare the viral infections you know with the common bacterial infections you see in practice (e.g. abscesses, wound infections and urinary tract infections). Think about the similarities and differences between bacterial and viral infections.
Joan Faoagali qualified as a specialist pathologist in Dunedin, NZ, and held roles including Director of Microbiology at Christchurch Hospital, Royal Brisbane Hospital, and the Princess Alexandra Hospital. She held a high interest in the prudent use of antibiotics, prevention of cross-infection, vaccination and the appropriate use and interpretation of pathology tests. She sadly died in January, 2017.