Agents of disease - stuff that makes you sick

I hate feeling sick. I’m one of those whingy blokes that always feels worse than symptoms may suggest. Yes, I’m a firm believer in the man-flu.

Disease is a condition an organism has or contracts that causes it to function at a less than optimal level. This disease causes a negative change in an organism’s state of health. Disease can be placed into two broad categories; infectious and non-infectious. An infectious disease is something that can be caught or transmitted, while, by definition a non-infectious disease cannot be transmitted. Non-infectious disease includes things like some cancers and genetic disorders. I will discuss non-infectious genetic disease or disorders during Unit 4. Remember that some cancers can be transmitted and are infectious, such as the Human Papillomavirus (HPV) that can lead to cervical cancer, and this is why it is important for everyone, guys and girls, to have the HPV vaccine. Why guys? What is the point of immunising everyone for this disease except for the carriers?

This rant will focus on agents of disease – the things that make you sick. The term for something that causes an infectious disease is pathogen. These are usually living, such as bacteria, but can be non-living, such as prions. I’m not going to get into the ‘are viruses living or non-living debate’ here.  I believe they are living but that is for another day and another blog. I’m going to divide pathogens up in this blog into micro-organisms, including bacteria, protists and fungi, macro-organisms, such as arthropods, annelids and nematodes, and non-cellular agents that will include viruses, viroids and prions.

Micro-organisms
Bacteria

Bacteria are an amazing group of organisms. The majority of life on this planet is bacterial. It was here before all other life and I believe it will be the last form of life to finally die off on this planet, many billions of years into the future. Bacteria inhabit just about every conceivable niche this planet provides, from kilometres under the surface within rock, to near-boiling hotsprings, in hyper-saline solutions and environments without oxygen, from the Antarctic to inside your gut. Yes, your gut. We are just another habitat for some bacteria, and most of the time we get along in a happy symbiosis. Bacteria is so good at surviving that some astrobiologists have suggested that life on Earth originated from Martian bacteria getting to Earth in a meteor ejected by some Martian supervolcano. While there is no evidence for this - there is no evidence that life once occurred on Mars in the first place - it’s still a cool little theory. The trouble with talking about bacteria as a group is that they’re not a homogenous group. There is more diversity in what we call bacteria than is all other forms of life. In general terms they are divided by shape: spherical cocci, rod-like bacillus, and spiral-shaped spirochaetes. But this is like saying a tall people are in one group, fat people in another and a third group made up by short red-heads. These gross morphological similarities do not necessarily mean they are related. Another way they are categorised is in the composition of the cell wall and how these respond to different types of dyes and stains. Those that can be stained are called Gram Positive, those that can’t Gram Negative. The final way bacteria can be grouped is by gaseous requirements: those that require oxygen (aerobic), those that thrive in the absence of oxygen (anaerobic) and those that don’t really care either way (facultative). It is a huge group that is all clumped together. The one thing that separates them from everything else is they are all prokaryotic. Despite our near pathological fear of bacteria, most bacteria leaves us alone or is beneficial and there are relatively few bacteria that make us ill. I mean there is so much of this stuff everywhere, it’s all over our bodies, the pillows we place our heads on, the utensils we eat with, if bacteria was ‘bad’ we would be in deep, deep trouble. As a rule, bacteria becomes a problem from a human point of view when in colonises a new habitat where it normally does not occur. An example of this may be bacteria entering a cut and forming a colony within the wound, or bacteria that is normally found in the lower intestine getting into the gut, usually by drinking water contaminated by faecal matter. Or not cooking food properly. There is a bacterium called Campylobacter sp that normal occurs in the intestines of chickens and causes them no problems at all. In fact, it is beneficial for chickens. The problem for us occurs in the processing of chooks for meat. Sometimes the meat becomes contaminated and, if you don’t cook your food properly, this bacterium is introduced into your gut. Where it will make you very sick.  So how do these microscopic organisms actually make us sick? One of the key factors is the virulence of the pathogen and how quickly they reproduce. In lay terms virulence refers to how good bacteria is at spreading and making you sick. The more virulent a pathogen is the better it is at making you sick. You need to remember here that bacteria aren’t trying to make you sick, that is not their purpose. They aren’t aware of you as anything other than habitat. Some bacteria make you sick by actively invading and destroying cells. Salmonella typhimurium will do this. We get Salmonella sp from contaminated food, and refer to its effects as food poisoning. It survives the acid of our gut and enters our colon where it destroys the epithelial cells of the intestine causing diarrhoea and vomiting. Another way that bacteria can make us sick is via their waste products. Like all life, bacteria eat and produce wastes. Some of the wastes from some bacteria are toxins and these make us sick and trigger responses in our bodies such as fever. As the bacteria colony grows and multiplies, the amount of toxin produced increases exponentially causing the host to become sick.

Protozoans - amoebas and their friends

In general terms, protozoans such as amoeba make us sick in a similar way to the bacteria previously mentioned. This Kingdom, again extremely diverse, is comprised mainly of single celled eukaryotes. The two examples I will give of disease caused by these protozoans is amoebic dysentery and ciguatera poisoning. Amoebic dysentery is caused by the amoeba Entamoeba histolytica. Humans are the host of this amoeba and it is spread by drinking water or eating food contaminated by the cysts of this protozoan. It is much more common in developing nations. It is nasty, I’ve had it. Many years ago when travelling through Nepal I made the mistake of drinking from a mountain stream high in the Himalaya. The problem was this stream was contaminated by this amoeba. Nepal is a developing country and no matter where you go you will find people up to, and beyond, the snow line. There were no roads where I was, I had walked for days to get there. Needless to say there were no flushing toilets either. Human waste is transported by rain or snow melt into the streams and carry the amoeba cysts with it. And that’s how the cysts got into me. I was luckily back in Kathmandu when the cysts hatched and the amoeba multiplied. It affected me in a similar way to the Salmonella sp mentioned before, eating away at the cells lining my intestine and has the same impact on your body. I lost many kilograms over several days before I was able to crawl into a rickshaw and get to a doctor. Holidays...
To go off on a wee tangent, my favourite species name of all time belongs to an amoebia. It is called Chaos chaos.

Another protozoan that can make you sick is the dinoflagellate Gambierdiscus toxicus, which is responsible for ciguatera poisoning. Dinoflagellates are single celled marine plankton. These are ingested by small fish at the base of the food chain and here, it is not the plankton that causes illness it is the toxins produced by it. These toxins are bioaccumulative and are magnified as they are passed up the food chain; small fish is eaten by big fish which is eaten by bigger fish until you catch yourself a large snapper or coral trout for dinner. You then ingest the toxins – which are heat resistant and not destroyed by cooking and you become sick yourself.

Apicomplexans are another protozoan that causes the disease malaria. I’ll discuss this later when I talk about vectors. Maybe, time and space dependent.

Fungi

I’ve spent too long talking about stuff I wasn’t going to mention so I will keep the last of the micropathogens, fungi, to a short paragraph.  Ringworm, athlete’s foot and thrush are fungal infections they colonise the external surface and digest your dead skin cells. External environment? Thrush? Thrush often occurs in the vagina and the mouth. Technically these are external environments. Vagina, etymologically speaking, means a fold and that’s what a vagina is. It is an envagination of the skins surface. These infections occur when a colonising cell reproduces successfully and produces many thousands of daughter cells. Enough said.

I’ve just looked at the word count and hit 1,500 so I’m going to skip over macro-organisms briefly. I’ll write another short blog about multiple-host life cycles of things like tapeworms tomorrow. Pathogenic macro-organisms are things like pubic lice, ticks, tapeworms... they are varied as are the effects and the impacts. They can be internal or external parasites. I’ll leave the details to your teacher. An interesting note though is that while rates of sexually transmitted bacterial infections such as Chlamydia are rising rapidly in the community, rate of pubic lice infections have decreased. The reason suggested? The popularity of Brazilians.

Non-cellular pathogens

Finally: non-cellular agents of disease. Viruses, viroids and prions. These pathogens are all non-cellular and technically non-living – remember that cells are the smallest independently functioning unit of life. I’m not going to argue that viruses are living here.

Viruses
Viruses are comprised of nucleic acid (DNA or RNA) surrounded by a protein coat. Some of these are further covered in a modified membrane. Smallpox, herpes and warts are examples of DNA viruses, hepatitis, influenza and HIV are examples of RNA viruses. Usually DNA viruses are double stranded but can be single and RNA viruses are single stranded but can be double. Viruses are obligate intracellular parasites. This means they must infect a host cell to reproduce. As they lack ribosomes and other organelles they hijack these in the host cell and force the cell to make viruses. How they do this is pretty cool. They insert their genetic information into the host cell and this interrupts the cells normal functioning. Two things can happen: the genetic information is spliced into the host’s cells genome where it lurks until later OR it forces the cell to make many, many virus particles until the cell eventually undergoes lysis (bursts) spreading these to infect new cells. I’ll use herpes as an example. The common cold sore is caused by the herpes virus. For example, if you kiss someone with a cold sore the virus passes to you where it inserts its information into your lip’s cell’s genome where it lurks. As your lip cells under mitosis and divide, the virus is also copied. This is called a lysogenic cycle. Then as a result of some trigger, your infected cells start producing virus particles, swell and burst. This is the lytic cycle and results in cold sores.

Viruses mutate and evolve regularly and rapidly, changing the markers on the outside of them to confuse your immune system. This is why it is hard to build immunity to some viruses such as influenza. Remember a few years ago we were worried about bird flu and SARS? Viruses can and do mutate and jump species. Fantastic evidence for evolution. It IS evolution.

Viroids are small virus-like pathogens that infect plants. That’s all I’m saying about them.

Prions

And to end this we have prions. Prions are abnormal proteins that change, denature, normal proteins by changing their shape. Eventually this leads to cell lysis which spreads the prions to infect other cells. Prions are usually found in neurons. The most well known prion disease is spongiform encephalitis. This disease causes huge holes in your brain (spongiform) and is commonly known as Mad Cow Disease, the only disease to be named after my ex-wife. This disease became a huge problem in England when infected sheep’s brains were mushed up and fed to cows, and then in turn fed to people. And you thought cows were vegetarians... Other prions disease include Kuru, a disease from PNG that came from eating the brains of dead relatives, and Creutzfeldt-Jakob disease.

Google these for more information. My pizza has arrived and I’m hungry. Hope this helps. Sorry it’s so long.
Watch a couple of these for giggles:
HIV life cycle. Pretty cool 
Bacteria 
Virus