C17L9 Pathogens

Antibiotic Resistance

Antibiotic resistance and virulence are two characteristics that make microbes more dangerous to humans. Actually, since virulence is defined as the relative ability to cause disease, it includes several different characteristics, but we'll talk about that later. For now, let's start by talking about antibiotic resistance, which occurs when a microbe acquires a gene that allows it to resist the effects of an antibiotic.

Since the 1930s, when the first antibiotics were discovered, modern medicine has grown to rely on these drugs more and more. Antibiotics were used to treat obvious bacterial infections. Then they started being used in cases of suspected bacterial infections, and now some doctors prescribe antibiotics for diseases that are almost certainly viral even though antibiotics aren't effective against viruses at all.

Some doctors may be covering all of their bases in case they're wrong and the disease is caused by bacteria. Others may be prescribing antibiotics to make the patient feel like they've gotten help from the doctor. And then there are doctors that have become so used to prescribing antibiotics that they do it almost automatically. After all, they're busy people, and it's hard to figure out what the exact cause of many illnesses is without doing a full workup and laboratory testing. It would be a waste of resources to hunt down the exact cause of every disease that they saw. Instead, it's easier to prescribe an antibiotic in case it's bacterial.

Different species of bacteria can exchange DNA, passing on antibiotic resistant genes

Why Antibiotic Overuse Is Bad

The problem with using antibiotics so often is that when antibiotics are always present, bacteria evolve resistance to them. Worse still, bacteria can pass along resistant genes not only to their direct descendants but to unrelated bacteria as well, even bacteria of different species! You see, unlike animal cells, bacterial cells exchange DNA with each other from time to time. In addition, bacteria can pick up DNA from the environment and use it themselves, so when an antibiotic-resistant bacterium dies and its DNA is floating around in the environment, another bacterium can pick it up. If the DNA is useful and gives the bacteria an advantage, then bacteria with that DNA will out-compete bacteria that don't have this competitive advantage.

So, in an environment where antibiotics are usually present, once one type of bacteria, even a harmless one, evolves resistance to an antibiotic, it's only a matter of time before it passes this resistance gene along to a pathogen. And the more antibiotics that these bacteria encounter, the faster the resistance genes spread. But over-prescription of antibiotics isn't the only cause of antibiotic-resistant bacteria.

Overuse of Antibiotics on Farms

Many farms across America routinely feed antibiotics to their livestock even when there are no signs of disease. In fact, about 80% of all antibiotic usage in America occurs in agriculture. This means that farms are using more than four times the amount of antibiotics used by hospitals, research institutions, and biotechnology and pharmaceutical companies combined, making these farms perfect breeding grounds for antibiotic-resistant bacteria.

Agriculture is responsible for 80% of all antibiotic use in America

So far, pharmaceutical companies have been able to stay one step ahead of antibiotic-resistant bacteria and develop new drugs before bacteria develop resistance to the old ones. But it is currently a biological arms race, and if we ever get to a point where we run out of new antibiotics, staph infections, gangrene, syphilis and many other diseases will again start claiming more lives.

Virulence

Switching gears a little, you may remember that a microbe is considered to be pathogenic if it causes any kind of disease at all or has the ability to cause disease under the right circumstances. Severity of the disease and our ability to treat it does not change how pathogenic a microbe is because it's an all-or-nothing term. If it can cause disease, it's a pathogen, and if it can't cause disease it's not a pathogen.

However, there is another way to measure how dangerous a pathogen is, and that is by how virulent it is. Virulence is the relative ability to cause disease. Virulence can be used to compare how dangerous or aggressive different pathogens are. For example, the swine flu strain that caused the Spanish Flu Pandemic of 1918 was much more virulent than the predominant flu strain of the year before because it was able to make more people sick and, more importantly, caused far more deaths.

Virulence Factors

Many characteristics can make a pathogen more virulent, including a life cycle that kills host cells, a rapid growth rate, production of toxins, interfering with normal functions of the host, the ability to evade the immune system and resistance to antibiotics. Some of these characteristics are easier to isolate and define than others. For example, when a pathogen produces a particular toxin that is harmful to the host or becomes resistant to a particular antibiotic, the genes and proteins responsible can be isolated and characterized. These proteins that make pathogens more virulent are called virulence factors, and the genes that encode for virulence factors are called virulence genes.

Now, just because a bacteria or virus carries a virulence gene does not necessarily make it a pathogen. Virulence is a scale, and some virulence factors play a bigger role in pathogenicity than others. In addition, virulence of pathogens is very species-specific. Let's take a look at some real-world examples of species specificity and virulence.

Virulence and Species Specificity

Virulence can be species-specific, as in the case of SIV

SIV, or Simian Immunodeficiency Virus, is very similar to HIV. In certain species of monkeys it causes a disease that is almost identical to AIDS and is extremely virulent, but it's not virulent at all to humans because it doesn't infect humans. In this case, SIV is a very virulent virus in some species of monkeys, but a harmless virus to us. It can also work the other way, though. Humans first contracted HIV from chimpanzees, but HIV infects chimpanzees without causing any signs of disease. It is not known exactly why HIV is so virulent in humans but is apparently not virulent at all in chimpanzees. However, it is fairly common for viruses to be non-pathogenic to their primary host species, which makes sense.

If a virus can live in a host without causing disease and killing the host, it will be able to reproduce and spread much more efficiently than if it killed its host within in a week or two. This is probably the main reason why Ebola virus doesn't spread very well through the human population. Ebola virus is perhaps the most virulent virus to humans. It kills over two-thirds of the people it infects, and it does it very quickly, usually in less than two weeks. However, killing a high percentage of people so quickly doesn't give the virus too many opportunities to spread very far, so outbreaks are usually contained to small areas and quickly die out.

Lesson Summary

Let's review. Antibiotic resistance is when a microbe acquires a gene that allows it to resist the effects of an antibiotic. Over-prescription of antibiotics and the routine use of antibiotics for farm animals that don't show signs of disease creates a selective pressure for antibiotic-resistant strains of bacteria. The more that antibiotics are used, the greater the selective pressure and the faster that resistance develops and spreads. Because bacteria are able to exchange DNA with other bacteria and pick up DNA from their environment, resistance genes can be spread very quickly, and even between bacterial species if there is a selective pressure. Pharmaceutical companies are currently in an arms race with pathogens to develop new antibiotics before bacteria can develop resistance to existing antibiotics.

Microbes that have the ability to cause disease, no matter how mild or severe, are called pathogens. Virulence is defined as the relative ability to cause disease, and can be used to compare how dangerous or aggressive different pathogens are. Many characteristics can make a pathogen more virulent, including a life cycle that kills host cells, a rapid growth rate, production of toxins, interfering with normal functions of the host, the ability to evade the immune system and resistance to antibiotics.

Proteins that make pathogens more virulent are called virulence factors, and the genes that encode for virulence factors are called virulence genes. Just because a bacterium or virus carries a virulence gene does not necessarily make it a pathogen. Virulence is a scale, and some virulence factors play a bigger role in pathogenicity than others. In addition, virulence of pathogens is very species-specific, and many viruses are pathogenic to some species and harmless to others. In fact, it can be advantageous to the virus to infect its host without causing disease, because a living host can spread the virus faster than a host that dies very soon after being infected.