What is a urinary tract infection?

The main urological associations define a UTI as a clinical picture of acute inflammatory symptoms of the urinary tract (burning or pain during urination, pain in the lower abdomen, frequent urination, feeling of not having voided properly, blood in the urine, etc.) accompanied by a diagnostic test confirming the presence of inflammation (urine dipstick and/or urine sediment, and preferably a urine culture or other test confirming the presence of inflammation (urine dipstick and/or urine sediment).) accompanied by a diagnostic test confirming the presence of inflammation (urine dipstick and/or urine sediment), and preferably a urine culture or other test confirming the presence of uropathogenic germs in sufficient quantity. Thus, although the "classical" urine culture is the standard test to diagnose urinary tract infections, sometimes special culture media or molecular biology techniques are needed to detect certain "atypical" germs that do not grow in the usual culture media (certain bacteria or some viruses). Therefore, both scenarios must necessarily be present, i.e. a germ "attacking" the wall of the urinary excretory system, and an immune system response leading to acute inflammation. This inflammation will cause the typical symptoms of urinary tract infection. It is important to stress this point, as many people suffer from what is called "asymptomatic bacteriuria". These are people who chronically have bacteria in their urine, but without these germs attacking the tissues, and without this causing any kind of inflammatory reaction from the bladder's immune system. In the elderly population, asymptomatic bacteriuria may be present in more than half of people. These are usually people who complain that their urine smells strong, without presenting any symptoms. In these cases, clinical guidelines advise against giving antibiotics as a matter of course, and to reserve them only for those cases where real symptoms are present. Some studies have even shown that people with asymptomatic bacteriuria are to some extent "protected" by these bacteria and have a lower risk of developing a symptomatic urinary tract infection. Even the use of non-uropathogenic strains of Escherichia coli as probiotics for instillation into the bladder. Unfortunately, these recommendations are often ignored by healthcare professionals and many people receive recurrent antibiotic treatment for this reason, which, in addition to being useless, encourages the emergence of bacterial resistance.

     I would like to clarify that the word "cystitis" is not equivalent to "urinary tract infection", although many people, including myself, use both words interchangeably, as many people are more used to the term "cystitis" than "urinary tract infection". Cystitis" actually means "inflammation of the bladder". However, a urinary tract infection can occur in the bladder (which is most common) or in other parts of the urinary excretory system (the kidney - pyelonephritis - or the urethra - urethritis - for example) and still be a urinary tract infection even if it is not cystitis. It should also be noted that inflammation of the bladder can be caused by germs as well as by other non-infectious agents (e.g. radiotherapy, certain drugs or autoimmune reactions). A lower urinary tract infection or infectious cystitis should not be confused with non-infectious cystitis. Non-infectious cystopathies, such as interstitial cystitis or radicular cystitis, are chronic inflammations of the bladder where the presence of an infectious pathogen has not been confirmed. Although there is growing suspicion that a large part of these chronic cystopathies may be related to infections by germs that do not grow in classical cultures or by intracellular germs, there is currently insufficient scientific evidence. 

Facts about UTIs

Every year, millions of medical consultations worldwide are related to UTIs, most of them in women, and UTIs are the most frequent cause of infection in outpatients. Statistics suggest that one woman in two will experience at least one urinary tract infection in her lifetime. In the USA, it is estimated that there is a prevalence of 11% (just over one in ten women) in the general population. With the exception of a peak incidence in young women (aged 14-24 years), probably related to the onset of sexual activity, this prevalence increases with age; thus, in women over the age of 65 years, it is thought to be about 201 PT3WT (one in five). 

     These figures are only an estimate, as they do not take into account the very large number of people who have not seen a doctor and who often take antibiotics or other treatments on their own and without professional supervision. 

What are repeat urinary tract infections?

A person is said to suffer from recurrent UTIs when they have three or more episodes per year, or two or more episodes in six months, confirmed by positive urine culture or other comparable technique. They are a great burden for the sufferer, causing a significant deterioration in quality of life. They are also a very serious public health problem, as, in addition to the serious complications they can cause in some cases (a serious kidney infection called pyelonephritis, sepsis or the formation of urinary stones, among others), they generate significant medical costs and large financial losses related to absenteeism from work. They can be particularly problematic in children, as in some cases they have been linked to long-term impairment of kidney function or to the development of high blood pressure, especially in the case of pyelonephritis (kidney infections). In addition, the high prevalence of these infections predisposes to overuse of antibiotics in some cases, which may lead to the development of resistance of some microbes to antibiotics. This further complicates the picture.

Germs implicated in urinary tract infections

Although theoretically any micro-organism can cause a urinary tract infection (bacteria, viruses, fungi, parasites, etc.), in practice, the vast majority are caused by bacteria that we call "uropathogens", i.e. bacteria mainly of intestinal origin, which possess certain pathological characteristics that allow them to easily colonise the urinary system.

     Within this group of uropathogens, the most frequent germs are:

• Escherichia coli: is considered to cause about 75%-80% of uncomplicated urinary tract infections. As I have already mentioned, it has the particularity of having "hairs" on its surface called pili or fimbriae, which allow it to adhere to the proteins on the surface of urothelial cells, mainly uroplakins. For this reason, it is the bacterium that most easily colonises the urinary tract. Moreover, there is increasing evidence that this bacterium is able to enter the interior of urothelial cells and remain hidden there, reactivating itself some time later to cause reinfection, known as QIR (quiescent intracellular reservoirs). This is another reason for its great capacity to cause repeated urinary tract infections. It should be noted, however, that within the genus Escherichia coli there are many different strains, only some of which are uropathogenic, many of which are harmless to humans. Returning to the case of people suffering from asymptomatic bacteriuria, the most frequently isolated germ is Escherichia coliIt should be noted that colonisation of the bladder by non-uropathogenic strains is not only not dangerous, but may to some extent protect against colonisation by more aggressive strains.

• Klebsiella pneumoniae: the second most frequent, albeit far behind E. coliis responsible for 6% infections. It has fimbriae as well. 

• Staphylococcus saprophyticus: together with Klebsiellais second in frequency, with another 6%.

• Enterococcus species: the fourth most frequent type (5%).

• Other common germs are: Streptococcus group B, Proteus mirabilis, Pseudomonas, Staphylococcus aureus, Candida species o Adenovirus type 11.    

It should be borne in mind that, depending on the region of the world, these percentages can vary considerably, although in the majority of cases E. coli is still the most frequent pathogen. For practical purposes, since recurrent urinary tract infections are usually caused by uropathogenic bacteria, I will focus on these bacteria and will leave out other pathogens as they are much less frequent. Before concluding this article on the most common bacteria, I would like to give a brief explanation of what nitrites are. I know many people who perform urine self-tests (test strips) on their own, which are available over the counter in pharmacies. This can be dangerous if you do not know how to interpret the results. Often, when bacteria are present in the urine, whether it is an infection or simply asymptomatic bacteriuria, the "nitrite" box on the test strip can be positive. Although I may seem heavy-handed, I have to stress that this result is not equivalent to having a urine infection. Nor would a negative result exclude it. In fact, nitrites are a substance that some bacteria are able to produce from a normal component of urine, nitrates, thanks to an enzyme called "nitrate reductase". Not all uropathogenic bacteria have this enzyme. For example, Staphylococcus saprophyticus y Enterococcus species do not. Therefore, a cystitis caused by one of these two bacteria will be negative for nitrite in the urine dipstick, but still be a cystitis. Likewise, a positive result for nitrites will not equal cystitis, but simply means the presence of nitrate-reducing bacteria in the bladder, even if it is asymptomatic bacteriuria. Moreover, the transformation of nitrates to nitrites takes a few hours (about six hours). Therefore, if the urine sample has been in the bladder for only a few hours, we may not get a positive nitrite result, even if nitrate-reducing bacteria are present in the bladder (such as E.coli for example). This is why it is recommended that the dipstick be done, if possible, with a sample of the first morning urine, which will have spent more time in the bladder. And my personal recommendation is not to use them unless you know how to interpret them correctly.

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