1)    Introduction

In the seventeenth century, an amateur microscopist, Antony van Leeuwenhoek, was the first to described bacteria. Later on, presence of germs in the air was proved by Louis Pasteur and John Tyndall. In the nineteenth, many scientists proved that disease arise from microorganisms. Robert Koch was the first to provide direct evidence that bacteria caused disease. He was studying anthrax at the time. Since then, hundreds of pathogenic bacteria have been associated with a disease.

Pathogenic bacteria are defined by the capability of causing disease in a host. To achieve this goal, pathogens require virulence determinants. This article will briefly explain what makes a bacterium pathogenic and what a pathogen must accomplish to cause disease.

2)    What makes a Bacterium Pathogenic?

Pathogens can be referred to as primary or opportunistic. Primary pathogens are regularly associated with disease in normal susceptible host. In the other hand, opportunistic pathogens cause disease in compromised host. Normal and compromised host are defined based on the states of their immune system. Compromised host tend to have a “weak” immune system such as infants, elderly, HIV-infect people and individuals already infected. Pseudomonas aeruginosa, a soil bacterium, is the best example of an opportunistic pathogen. P. aeruginosa infects burn and other wound and is a major problem in hospital. A death rate of 60% can be associated with outbreaks in burn units.

Pathogenic bacteria are often referred to specific species but strains within a given specie can have a different ability to cause disease, which is known as virulence. Level of virulence can be measure. In a particular specie, a strain can be highly virulent, weakly virulent or avirulent (inability to cause disease). The best example is our good friend Escherichia coli. The commensal strains of E. coli can happily lived in our intestine without causing us any harm whereas enterohemorrhagic strain of E. coli (EHEC) such as strain O157:H7 can cause deadly infection.

What makes a bacterium pathogenic is the set of gene that it possesses. The genes can be distributed on the chromosome, on plasmids (independent extrachomosomal genetic elements) and prophage A prophage is a bacteria virus (bacteriophage) in which the viral genes are incorporated into the bacterial chromosomes without causing disruption of the bacterial cell and is latent. Recently, blocks of virulence genes in the chromosome have been observed in virulent strain but not in avirulent strain. For example, commensal E. coli does not possess these blocks whereas EHEC does. These blocks are named pathogenicity islands and significant differences (e.g. nucleotides composition (%GC)) between the chromosome and the island is observed and typical. The differences suggest that mobile genetic elements may have played a significant role in the evolution of pathogens