Pathogenicity: Microbial Virulence

Worldwide, infectious diseases account for more than 10 million deaths each year (World Health Organization, 2004). In the past decade, exciting developments in molecular genetics have allowed scientists to explore the complex pathways used by microorganisms to elicit these diseases. So, what do we know so far?

Back in the time of Hippocrates, when microorganisms were virtually unimagined, doctors believed that disease stemmed from an imbalance in the four bodily fluids: blood, phlegm, yellow bile, and black bile. Then, in the 1670s, Dutch cloth merchant-turned-microbiologist Antonie van Leeuwenhoek offered the first observations of "animalicules" swimming in droplets of rainwater magnified under his microscope. But it wasn't until the late 1800s that early microbiologists began putting two and two together, eventually determining that microorganisms cause disease. But how does this happen?

Unlocking the genetic code of disease-causing microorganisms has helped identify common features associated with their virulence. These include pathogenicity islands, the incorporation of bacteriophage genes, and abnormal amounts of guanine and cytosine in the sequences of virulence genes.

Of course, in such discussions, it is important to note that "typically" is the operative word. In fact, normally avirulent strains of E. coli can—and do—cause disease when they wander into the wrong places at the wrong times. For example, many women have experienced the painful burn of a urinary tract infection when commensal E. coli from the digestive tract takes a wrong turn up the urethra. Such phenomena raise another important point: Identifying the virulence genes encoded by notorious pathogens (like E. coli O157:H7) has been only part of the microscopic puzzle. Over the years, researchers increasingly began to notice that some organisms cause disease only when they appear at certain sites at certain times, whereas others cause trouble no matter where and when they turn up. Thus, the other major part of the puzzle involved explaining and predicting why this happens.

Consider this: Candida albicans generally colonizes the skin and mucous membranes without much difficulty. However, this fungus can cause vaginitis, nail infections, and even toxic shock if the mucosal barrier—of the genital tract, for example—is disrupted, the immune system is suppressed by drug therapy, or someone is exposed to an especially large inoculum. In fact, in 1969, German physician Wolfgang von Krause imbibed a concoction of approximately 1 billion C. albicans to prove that the normally docile fungus can indeed cause disease. Unfortunately, he was correct (Krause et al., 1969).

This realization introduced the concept of opportunistic pathogens, or organisms that typically only cause disease under certain predisposing conditions. Do these organisms encode virulence genes in the traditional sense? That question remains the subject of much controversy among modern microbiologists. Sometimes the answer is yes, but often the so-called virulence genes encoded by these organisms are as unassuming as metabolic genes that allow growth on certain substrates. It just so happens that, in the absence of an immune response, these organisms are free to grow and replicate without restraint. The reaction (or lack thereof) of the host to the pathogen is thus an important component of infection.

This controversy led to the development of a molecular version of Koch's postulates (Falkow, 1988, 2004). The original postulates were developed in the 1870s by Robert Koch, a country doctor faced with more farm-related anthrax cases than he could handle. Koch purchased a microscope for his clinic and noticed large organisms swimming in the blood of his infected patients. After isolating the bacteria in a makeshift lab, Koch realized he could elicit anthrax by injecting animals with pure cultures of the bacterium, eventually known as Bacillus anthracis. The doctor's technique—extracting the suspected agent of disease, growing it in pure culture, injecting the purified agent into an animal, and subsequently isolating it from the diseased animal—became known as Koch's postulates. Koch later used this process to identify the causative agents of cholera and tuberculosis, and his postulates laid the groundwork for modern studies of virulence.

Today, researchers know that the pathogenicity of many pathogens—pathogens that fail to fulfill the postulates and otherwise might be rejected—plays a causal role in disease and infection. It is important to note that Koch's postulates provide scientists with a framework to guide research. New technologies and information, including the genomic sequences of many of these pathogens, now offer a more complete picture of the requirements of pathogenicity.

The field of microbial genetics has come a long way since Van Leeuwenhoek's first renderings of microorganisms in the 1600s. Thanks to recent advances in genomics and proteomics, scientists have learned that the genes involved in virulence tend to share similar properties. Many of these genes are encoded on pathogenicity islands, bacteriophages, or insertion sequences nestled within an organism's chromosomal DNA. Other so-called virulence genes can assist with metabolism and thus play an inadvertent role in pathogenicity by allowing otherwise commensal organisms to flourish in warm, moist environments such as the human body. Once identified, these genes can be further examined to reveal the complex pathways between gene expression within virulent microorganisms and the development of disease.