Inappropriate antibiotic use is one of the main causes of the spread of drug-resistant bacteria. Methicillin-resistant Staphylococcus aureus (MRSA) is a notorious example of this, and one with which most nurses are all too familiar. Others include vancomycin-resistant enterococci, extended-spectrum beta-lactamase-producing coliforms, and resistant varieties of a range of other species.
The existence of such bacteria does not mean that antibiotics should no longer be used; if they are used correctly they are still one of the most potent weapons against bacterial infections. They are not, however, effective against viruses.
I would be delighted if, when I asked friends and relatives how they are they replied: ‘Oh, awful! I’ve got a rotten cold; I think it might even be flu. I’ve been to the doctor but he wouldn’t even give me any antibiotics - just told me to take some paracetamol and said it would get better on its own. I think he just wants to save money.’ Such a reply would demonstrate that the message is getting through - at least to health professionals - that antibiotics are ineffective against viruses. Once my relatives were feeling better, I might even try to explain why I was pleased that they did not receive the desired antibiotics.
To understand why antibiotics are not effective against viruses it is necessary to understand what bacteria and viruses are, and how they differ. This may be difficult, because they seem similar in some ways - both are far too small to see with the naked eye, and both cause a variety of diseases.
Unfortunately, the words ‘virus’ and ‘bacteria’ are often used interchangeably with ‘germs’ and ‘bugs’ when infectious agents are being discussed, which makes it more difficult for patients to understand the differences. Furthermore, articles in the press referring to the ‘MRSA virus’ are depressingly common, because Staphylococcus aureus is a species of a bacterium.
It is essential that nurses understand the differences between bacteria and viruses, because once they realise how antibiotics work, it becomes clear why they are effective only against bacteria.
How do bacteria and viruses differ?
The most obvious difference between bacteria and viruses is their size. The existence of viruses was first detected when it was discovered that infectious extracts from diseased plants were still capable of causing disease after being passed through a filter fine enough to remove bacteria.
While both bacteria and viruses are too small to see with the naked eye, most bacteria are around one micrometre (one-thousandth of a millimetre) long and can be seen with a good optical microscope such as those found in most microbiology laboratories.
Viruses, on the other hand, can be as small as 30 nanometres across. (A nanometre is one-thousandth of a micrometre, or one-millionth of a millimetre.) This makes them smaller than the wavelength of visible light, which means that they can be ‘seen’ only by using an electron microscope. Their small size means they can easily be carried in the air or in droplets, as is the case with many viral diseases such as influenza or the common cold.
However, the most significant difference between bacteria and viruses is more fundamental. Bacteria are capable of an independent existence, and are found in virtually every environment on earth, from hot springs to ice caps.
Of the many thousands of species of bacteria known, relatively few cause disease. Although their structure is very different to that of plants or animals, they do have common characteristics: they consume nutrients, respire, excrete and reproduce independently.
Viruses, on the other hand have little in common with other life forms; indeed there is considerable scientific debate as to whether they should even be considered to be alive. They consist of little more than a package of genetic instructions, and the only way for a virus to reproduce is for it to invade a living cell and induce that cell to use the instructions to create more virus particles. Outside a cell, they are metabolically inert.
In some ways, a virus can be considered the ultimate parasite. There are viruses adapted to use the cells of animals, plants and even bacteria for reproducing themselves, and many cause diseases of greater or lesser severity while doing so. Table 1 Lists some important differences between bacteria and viruses.
How antimicrobial agents work
Micro-organisms are often susceptible to the same agents that harm other living things - heat or disinfectant chemicals such as bleach or alcohol, for example. However, susceptibility is not constant, and some bacteria are able to become spores that are extremely resistant to both heat and some chemicals. It is for this reason that autoclaves operate at temperatures above boiling point and why hand hygiene must involve physical removal of bacteria with soap and water rather than disinfection with alcohol gel when caring for patients who have Clostridium difficile infection.
Viruses also vary in their susceptibility, with ‘non-enveloped’ viruses such as norovirus being more resistant to alcohol than ‘enveloped’ ones, such as influenza. An ‘enveloped’ virus has an ‘envelope’ surrounding the viral particle that is generally derived from the host cell’s outer membrane. Some may have spiked structures projecting from the capsule. Both these structures are an aid to attaching to and infecting host cells, but enveloped viruses are generally more fragile than those that are non-enveloped.
The drawback to destroying micro-organisms with heat or chemicals is that these are of limited use in treating an infection, since what will harm the infecting organism will also harm the host. Antimicrobial drugs therefore operate on the principle of selective toxicity, exploiting differences between an infecting organism and its host.
The cell wall of a bacterium is an ideal site for the action of antimicrobial agents to attack the infecting organism while having a minimal effect on the host.
Antibiotics that stop cell wall synthesis include the beta lactams, such as penicillin, and glycopeptides such as vancomycin. They kill bacteria because, without a cell wall, osmotic pressure causes the bacteria to burst. However, an alteration in the target site can confer resistance of the bacteria to the drug. Functions in the bacterial cells, such as protein or nucleic acid synthesis, are targeted by some antibiotics but these functions are specific to bacteria, and so will have no effect on viruses.
Because viruses have no metabolism of their own, are inert while outside a cell and use the host cell’s own mechanisms for their reproduction, it is difficult to create drugs that will inactivate a virus without damaging the host. This means that strategies to combat viral diseases must concentrate on infection prevention through vaccination or infection control measures such as handwashing and food hygiene.
It is important that nurses understand the differences between bacteria and viruses because they need to be able to explain these to their patients. Being aware of these differences will help them to understand why a treatment that is suitable for one situation will not be appropriate for another, and why antibiotics cannot be used to treat a viral infection.