Major Organism Groups That Cause Disease In Humans Biology Essay


There are four major organism groups that serve as agents that will cause disease in humans. This includes viruses, fungi, parasites and bacteria (our focus will be on the bacteria). Bacteria belong to the prokaryotic kingdom. Prokaryotic organisms are the simplest unicellular organisms. They reproduce by asexual division. There are many size and shape of bacteria found in nature. Their size ranges from 1 to 20 micrometers (or larger). The common shapes of bacteria include rods, curved, cocci, and spirals. Humans are exposed to bacteria from the moment they are born. Human body serves a perfect place because it provides environment needed for bacteria to survive such as warmth, moisture and food. Bacteria have obtained the special capability to invade into host, stay in the niche, use all the resources and also be able to escape smoothly from the immune system. Bacteria are present in air we breathe to the food we eat. Most are a not harmful but some could produce severe disease. It is not possible to remove them completely. Some live transiently and others are present permanently in a parasitic relationship. Humans get infected with disease because of two main reasons. One reason could be the toxic effects of toxins produced by bacteria and other by invasion of bacteria into sterile sites of the body. The outcome of interaction between the human host and bacteria depends on three main factors: virulence of the bacteria, the site of exposure and the host's ability to combat against the attack. Therefore the result of this relationship could range from a cold to life-threatening disease. Basically if you were in a fight with an enemy then the one to win will be determine by fact that who has the best weapon, where the target is hit (heart or leg) and also how well one is able to retaliate from the wound.

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Most of the bacteria have a cell wall which makes them different from other parasitic agents. There are several ways to differentiate between bacteria. Cell wall can serve as an initial distinguishing factor. Two major broad groups of bacteria are gram positive bacteria and gram negative bacteria, so called because of their ability or inability to take up the gram staining. Cell wall is the outermost part of the bacteria. Although multilayered two distinct layer that make up a cell wall are outer membrane layer and a peptidoglycan layer. Peptidoglycan layer is unique to bacteria and is much thinner in gram negative than gram positive bacterium. It is a complex, intertwined layer that surrounds the entire cell. It provides rigidity and is helps maintain the shape of the cell. Due to its presence in bacteria only, it severs as a key target for antibacterial drugs. Some external features that could also be present on the surface of the cell wall are capsule, flagella and pili (present in some bacterium). Apart from the peptidoglycan layer, another unique feature that is present only in gram negative bacteria is the lipopolysaccharide, found in the outer membrane.

Lipopolysaccharide (LPS) also known as endotoxin, so called because they are embedded in the cell wall as oppose to exotoxins which are freely released from the cell. It is liable for many symptoms of the disease such as fever and shock (especially hypotension). LPS is composed of 3 units: Lipid A, core polysaccharide region and O antigen. Lipid A unit is the one responsible for the toxic effects. This unit is similar for all gram-negative bacterium. The core polysaccharide region is same for all bacteria. O antigen extends from the core to the outer surface of the bacteria. This unit provokes the immune system to act. It can used to distinguish different stains of bacteria in the laboratory. LPS is poorly antigenic therefore it is able to survive in the host for the longer time. Another features that make gram negative bacterium more virulent compared to the gram positive bacterium are: its ability to be resistant to lysozyme (lysozyme which is present in humans has the capability of breaking the glycosyl bonds of the peptidoglycan layer thus helping human prevent the infection) and its antibacterial activity of penicillin due to the presence of b-lactmases present in the periplasmic space. In addition presence of capsule in certain strains helps gram negative bacteria live longer because it poorly antigenic and antiphagocytic. Antigen serves as a identification card for all the bacteria so if one is poorly antigenic it is like carrying a fake id so no one will suspect that you are illegal in that region. This allows one to live in the area without being thrown out or getting killed by the security.

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F. tularensis is a very small coccobacilli negative gram bacterium. The normal size of the bacteria is 0.2 x 0.2 to 0.7 micrometer. It does not stain very well due a thin wall. It is an immobile organism that has thin lipid capsule which gives it its virulence nature by making it undetectable by the immune system during infection. This bacterium has a specific growth requirement. Most strains require the presence of cysteine and oxygen for its growth. Even though oxygen is poisonous for certain bacteria, F. Tularensis is strictly aerobic, that is, it requires oxygen for growth. It overcomes not so favorable characteristic for most bacteria because aerobic bacteria produce superoxidase dismutase and catalase enzymes which can detoxify hydrogen peroxide and superoxide radicals that are the toxic byproducts of aerobic metabolism. In order to detect growth of this type of bacterial cultural it takes about 3 or more days which could be harmful for patients that have a weak immune system.

Francisella has two main groups Francisella tularensis and Francisella philomiragia. Focus for this paper will be Francisella tularensis; its primary contribution is to cause tularemia in animals as well as in humans. In other words it causes glandular fever, rabbit fever, tick fever and deer fly fever. The four major subcategories of F. tularensis are: Subspecies tularensis (type A) and Subspecies holarctica (type B), Subspecies mediasiatica and Subspecies novicida (latter two are not of great relevance). The subspecies are divided based on their virulence and epidemiology. Type A is more virulent and more prominent in United States. Type B is less virulent in comparison to type A and is more prominent in Europe.

Understanding the divisions is of importance because it epidemiological factors for each region are unique and therefore their course of clinical disease is also drastically different. It aids the researcher or the healthcare professional to take action based on the individual factors that are affecting a particular population. F. tularensis subspecies tularensis (type A), relating to Tulare County of California where it was first described, is for the most part is limited in the North America. Where as subspecies holarctica (type B), holos meaning whole and arctos meaning northern region, is more spread over the Northern Hemisphere. Type A is further subdivided into type A-west and type A-east. Type A-west is found in region from the Rocky Mountains to the Sierra Nevada Mountains. Type A-east is found in Arkansas, Missouri, and Oklahoma and along the Atlantic Ocean. It is found over such wide area due to the import of infected rabbits in 1920s and 1930s. Type B strains on the other hand are predominately found along the waterways such as the Mississippi River and the areas that observe high rainfall such as the Pacific Northwest. This distribution is also based on the reservoirs and vectors of F. tularensis that are encountered in the region. The principal natural reservoirs for F. tularensis are small wild animals, domestic animals, birds, fish, and blood sucking arthropods. The primary vectors are usually ticks, mosquitoes and biting flies. The most common reservoirs and vectors linked with human disease are rabbits, cats, hard ticks and biting flies. Type A infection are due to contact with rabbits, hares, and cats as well as biting arthropods such as deer flies and hard ticks. Type B infections are related with rodent such as rats, mice, muskrat, beaver or squirrel and cats only.

As you can see from the graph on the left, there were few cases of tularemia in later part of the 20th century than in the earlier. This can be attributed to the fact that hunting might have decreased over the years therefore the exposure to the infected animals was less. Another reason could be development of antibiotics that would help with the disease. Additionally, tularemia is commonly unsuspected and highly complicated to detect and validate via laboratory tests. From the graph on the right, it is evident that most of the cases for infection were in summer. This can be attributed to the fact that there is higher exposure to ticks in summer. The incidents of infection in winter can be credited to hunters coming in contact with infected rabbits. Also, the chances of disease increases when the winters are warmer than usual and it is followed by wet summer because this gives chance to the tick populations to flourish.

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Several genetic trains are virulent factors which give bacteria an advantage surviving the in the host and/or causing disease(s). F. tularensis is an intracellular pathogen. That is it lives in a hideout in certain type of cell for a long time without being detected by the host immune system. F. tularensis can continue to exist for a long period of time in macrophages of the reticuloendothelial system. It does so by inhibiting the phagosome-lysosome fusion. This method is important for the bacterial survival because if phagosome and lysosome were to fuse than all the contents that are filled in the phagosome would be digested by the enzymes released by lysosome thus leading to its death. Pathogenic strains have antiphagocytic, polysaccharide-rich capsule. If the bacterium were to be stripped of its capsule might leads to its death because the capsule defends the bacterium from getting killed by the complement-mediated killing by the host's immune system. Like other gram-negative bacterium it also has endotoxin but is less active compared to other gram-negative bacterium. During the early stages of infection a strong innate immune response with making of interferon-g and tumor necrosis factor (TNF) are very important because it controlling the bacterial replication in macrophages. For late stages of disease a specific T-cell immunity is required for activation of macrophages for intracellular killing.

F. tularensis is an enzootic pathogen, that is, it is common in animals but humans are accidental host. Although not human to human transmission is possible other routes of infections are inhalation of the aerosol species of the bacterium whose primary target is the respiratory tract; ingestion of improperly cooked meat or contaminated water can affect gastrointestinal tract; abrasions or entry through unbroken skin are targeted by handling of infected animals or bite by blood sucking ticks. There are several forms of disease caused by F. tularensis.

Ulceroglandular tularemia: It is the most common type of tularemia. It is caused mainly by direct contact of skin with the infected animals or its remains or a bite from an infected tick. It causes cutaneous ulcer and swollen lymph nodes, usually armpits and groin. It starts as a small but painful papule that becomes an ulcer at the site of entry of bacteria (tick bite or direct inoculation of the organism into the entry). Glandular tularemia infection is caused by same method as ulceroglandular tularemia. The conditions will generate swollen lymph nodes does not cause ulcer for the affected patients.

Oculoglandular tularemia: It is caused by direction contamination of the eye (via contaminated fingers or aerosols). It causes swollen cervical lymph nodes. Symptoms include irritation and inflammation of eye and swelling of lymph glands in front of the ear. Painful conjunctivitis and regional lymphadenopathy also occurs.

Typhoidal tularemia: It does not have any explicit route of transmission. It causes systemic signs of sepsis i.e. the bacterium enters the blood and causes inflammation and affects the body systems.

Pneumonic tularemia: It is caused by inhalation of infectious aerosols. Another mode of transmission is when other form of tularemia is left untreated and the bacteria spreads through the bloodstream to lungs. It is the most dangerous form of tularemia having the highest amount of morbidity and mortality. It is extremely difficult to detect. It can used as a biological weapons causing destruction in large number.

Oropharyngeal and Gastrointestinal tularemia: Its primary cause is eating or drinking of contaminated food or water. Common symptoms include: sore throat, mouth ulcers, tonsillitis, and swelling of lymph gland in the neck.

As we can see from these different forms of disease symptoms vary depending on the route of infection. Some have localized reaction (swelling of lymph nodes or ulcers) based on the specific infection site (tick bite or skin abrasions). These could lead to systemic infection (normally caused by aerosols or ingestions) if left untreated for a long period of time.

Diagnosis of this bacterium is a not easy process because isolation of the specimen is not only difficult but unsafe for the laboratory staff and the physicians. Due to its small size it can enter through skin lesions and mucous membranes or it can be inhaled if aerosols are present. It is needless to say that extreme precautions must be taken both during collecting a specimen and while performing a test.

Confirmation of growth of F. tularensis by Gram-stain method is possible via collection of specimen from infected nodes or ulcers. But most of the time this method is ineffective because being a gram negative bacteria it does not stain very well as it has thin cell wall. Another obstruction utilizing this method is that it is dangerous even to handle it even in safe laboratory environment. A more effective way would be to use a direct staining method which uses fluorescein-labled antibodies directed against the bacterium. These could be purchased from Center of Disease Control and Prevention (CDC) as well as state public health facilities. Downfall of this method could be that the response from antibodies does not happen until few days after start of the disease.

Another method which can be more successful is a Nucleic-Acid-Based Tests, Polymerase Chain Reaction (PCR). This would give more accurate results for detection but is not available as readily. It could help distinguish between strains and could be extremely useful during bioterrorist attack. Unfortunately it is not very widely employed due to limitation of its availability, monetary cost and long period of developing an essay that works perfect with identification of bacterium. New methods have to be fairly easy to use so all the laboratory can use it efficiently, accurate, cost efficient, time efficient, and practical.

It is difficult to culture F. tularensis in a laboratory media due to its specific requirement of having sulfahydryl-containing substance, for example cysteine, for growth. On the other hand this is readily available in host's blood (human or animal). But laboratories have been able to grow its culture on specific agar known as chocolate agar or buffered charcoal yeast extract (BCYE) agar. This agar supplies sufficient cysteine for the growth of the bacterial culture therefore laboratories do not have to invest in special media for instance blood cysteine agar or glucose cysteine agar. It is very important to inform the laboratory or health care facilities that do conduct these tests if any infection with F. tularensis is suspected. Although this bacterium is small in size, it is highly virulent in nature and due to its slow growing process (3 to 4 days) it might go unnoticed. Additionally, laboratory must be informed because extra precautions have to be taken since laboratory workers can get infected easily during testing. No human to human transmission is possible but if the laboratory personal has a cut or aerosols are produced than they are high chances of him/her to get infected. One major difficulty while growing culture for F. tularensis it is extremely difficult to isolate it. So one might have a negative lab results even if the bacterium is present. The reason being the long incubation time requirement for example in blood cultures it requires about a week or more and cultures of respiratory specimens. This method should only be used for preliminary identification. Further confirmation can be obtained by its reactivity with certain antiserum. Based on the complexity, uncertainty and hazardous nature of this method another technique, such as PCR would be a better option to identify and detect F. tularensis.

Another process of choice for diagnosis could be antibody detection. The patients infected by Tularemia often show a fourfold or greater boost in the titer of the antibodies during the disease (1:160 or more). Them major drawback of this method is that antibodies such as IgG, IgM and IgA are present in blood even several years after. This makes it harder to distinguish between the past and present disease condition. Also, the antibodies targeted for another zoonotic disease, Brucella detection can also react with Francisella.

Apart from being difficult to diagnose its symptoms are similar to common illnesses therefore could be overlooked. Certain antibiotics do exist to combat this disease for example Streptomycine, Gentamicin, Fluoroquinolones (eg. Ciprofloxacin) and Doxycycline. Streptomycine is no longer an antibiotic of choice because of its high toxicity levels. Fluoroquinolones and Doxycyclines are choice of antibiotic when it comes to in vitro bacterial activity and mouse animal model. Penicillins and Cephalosporins are not useful because F. tularensis produce beta-lactams. Patients can recover fully if treated as early as possible (mortality rate is less than 4%) but could be fatal if left untreated for a longer period of time especially for type-A east strains, considering its virulence nature.

There are no special preventative measurements that need to be taken in order to prevent infection. People should be careful when dealing with reservoirs (rabbits, hares, cats, birds, fish, or blood sucking arthropods) and vectors (ticks, mosquitoes, biting flies). When going to the areas such as forest or woods where such exposure is unavoidable use insect repellent, wear long pants, long sleeves shirt, long socks to ensure that ticks and deer flies do not come in contact with your skin. If bitten by a tick then remove it right away. Avoid contaminated water and food. If handling animals that are sick make sure to wear gloves. If mowing or landscaping then it is important to wear a dust masks to prevent inhalation any aerosols and avoid mowing over carcasses. Live vaccines can reduce severity of the infection but cannot stop the disease in its entirety. Inactivated vaccines are ineffective because they do not elicit any immune response.