Bacteria - Basics Facts

What are bacteria?

Gram stain appearances of medically important bacteria

• Bacteria that manage to keep the original purple dye have only got a cell wall - they are called Gram positive.
• Bacteria that lose the original purple dye and can therefore take up the second red dye have got both a cell wall and a cell membrane - they are called Gram negative.

A table of the staining characteristics for some common bacteria of medical importance is given below. Note that for cocci, it is not just the shape and colour of the individual bacterial cells that is important, but the way that all these cells group together too. Put simply, round purple balls that look like bunches of grapes under the microscope (i.e. Gram-positive cocci in clusters!) suggests that the bacteria are staphylococci. Most of these bacteria are fairly flexible about the conditions they require for their growth - give them roughly the right temperature and a few simple nutrients and they are happy. A few of them are rather more fussy though and bacteria such as Clostridium and Bacteroides are examples of this; they are called anaerobes which means that they can only grow if there is absolutely no oxygen present.

Why should you bother with all this? There are three reasons:

• dividing up new information makes it easier to learn - and unfortunately there is no option but to learn the core material in this table.
• this is the language used by clinicians, especially when they are trying to manage seriously ill patients.
• knowing this information well makes it easier to understand bacteria, to appreciate the different sorts of infections they cause and the antibiotics that can be used to treat them.

Exceptions to the rule: not all bacteria are shown up by the Gram stain, but there are only a couple of important exceptions for you to remember now!

• Mycobacterium tuberculosis has a thick waxy coat that stops the bacterial cells taking up the Gram stain; if these bacteria are suspected, the specific Ziehl Neelson stain must be used.
• A small group of organisms, such as Chlamydia and Mycoplasma, do not have conventional cell walls at all and specialised techniques are often required to diagnose infections caused by these bacteria.

How do you get information about bacteria in clinical practice?

A number of different samples can be sent to the diagnostic laboratory for microbiological analysis including fluids (such as blood, urine or cerebrospinal fluid [CSF]), pieces of tissue or swabs taken from infected lesions. For specimens such as CSF that would normally be sterile, microscopy can be very useful as the presence of any bacteria is always abnormal. However, for the great majority of specimens, the sample will have to be spread out onto culture plates to grow the bacteria, to see if there are any in the sample that might be the cause of the infection - this will generally take at least 24 hours. If there are any suspect bacteria there, they will probably need to be identified further and also checked out to ensure that they are not resistant to the effect of antibiotics - at least another 24 hours. In situations where it is not possible to grow bacteria, it may be possible to diagnose infection based upon a person's antibody responses - but this is not usually a rapid method either! So that in many cases, you must make initial decisions about antibiotic treatment based upon a sound knowledge of bacteriology - what is the most likely or most important bacterial cause of this infection and what is the most appropriate antibiotic for treatment? This will usually be backed up by laboratory investigation to confirm your diagnosis (or not!) and help you refine your future management of the patient. To practice clinical medicine effectively you must have a good knowledge of bacteriology, some idea about the service provided by diagnostic laboratories and the ability to interpret the reports that are issued.

In the table of Gram stains above, the bacteria were grouped together and listed by genus name (plural = genera). This is a name given to a collection of bacteria that share many fundamental, major, obvious characteristics. However, by examining bacteria more closely, perhaps by looking at some of their biochemical capabilities for example, it is possible to divide them up further into individual species. This is very often of great clinical significance. An example of how this works in clinical practice is given below. Also, at the end of this chapter there is a table with a list of common bacteria of medical importance, the sites at which they can normally be found and the sorts of infections that they cause when things go wrong. This is intended to be a ready reference as you go through the course and you are not intended to learn all the information straightaway, but hopefully, by the end of the course, you will have become familiar with much of this information.

Case history:

This fairly simple case does illustrate a number of points:
The human body is not sterile - bacteria do not equal disease! Surfaces exposed to the external environment, such as the skin, vagina and upper respiratory tract, are teeming with bacteria, most of which are either harmless or positively beneficial. In terms of sheer numbers though, the digestive tract must hold the record - human faeces typically contains more than 10,000,000,000,000 bacteria/ml.

A clinical problem, such as a pyrexia, requires a provisional diagnosis before you can decide on the appropriate patient management. Infections of the respiratory tract, the urinary tract and the wound site are all common causes of a post-operative temperature, not to mention some of the non-infectious possibilities described above. Your initial decisions will probably be based upon clinical history and examination, but laboratory investigation is important as it allows you either to confirm your diagnosis or else modify your treatment when things are not going as smoothly as you had expected. A very large range of bacteriological investigations is available and you may not always know which are the most useful - the lab can help you there if you give them good clinical information. Microbiology takes time so that it is vital to get it right first time - what if the lab didn't know that this patient was allergic to penicillins and only tested flucloxacillin?

Clearly, if you want to get the best from the laboratory for your patients, good communication is desperately important.

Site of Infection and Colonisation by Some Medically Important Bacteria

NB: for some infections, e.g. with Salmonella , asymptomatic carriage of the organism may persist for some time, especially following infection. However, as this is generally only temporary it is not the same as true colonisation and has therefore been placed in brackets with a question mark in the above table.

© AJC 2007.