Chapter 4: Prokaryotic Cells and microorganisms
This chapter looks at the morphology, and pathogenic mechanisms of prokaryotic cells and microorganisms respectively. It discusses in detail the structural components, functions of the different organelles of prokaryotic cells as well as feeding and survival mechanisms of prokaryotic microorganisms.
Prokaryotic cells have a specialized cellular structure. Prokaryotes are unicellular and their cells have no cellular organelles, that is, a membrane does not surround their nucleus. These cells contain intricate phospholipids, sterols, and sphingolipids. They are typically 1-5µm in diameter .
Their outer surface is covered by glycocalyx, which is a gel like cellular envelope that provides prokaryotic cells with protection against harm (phagocytosis). They have a capsule that enhances their pathogenicity and survival in the environment.
The cell wall of prokaryotic cells is found just below the glycocalyx. It provides the organisms with a definite structure as well as protection against destruction from drug compounds. Prokaryotic bacteria are grouped into two depending on how their cell walls stain with the Gram stain. Bacteria whose cell wall stain violet or pink with the Gram stain are grouped as Gram positive bacteria while those that stain red are grouped as Gram negative bacteria. Thus, Gram-positive bacteria have a thick cell wall with peptidoglycan with a narrow periplasmic space. Their cell wall is more permeable but less susceptible to breakdown or destruction. On the other hand, Gram-negative bacteria have a thin layer of peptidoglycan on their cell wall. They have a wider periplasmic space, less permeable and more susceptible to destruction or breakdown. However, Gram-negative bacteria are more resistant to immune mechanism of the host and antibiotics than Gram-negative bacteria. The outer membrane is bound to the cell wall by lipoproteins. It is responsible for the different shapes of bacteria. Coccus bacteria are round in shape, bacillus are rod shaped, vibrio bacteria are slightly bent rods and spirillum bacteria are spiral shaped and cylindrical.The cell membrane is primarily composed of phospholipids. The cell membrane regulates movement of substances into and out of the cell. The internal structure is composed of cytoplasm with dissolved nutrients and waste products, a single circular loop of DNA, plasmids, 70S ribosome that is the site for protein production, and inclusion bodies. In addition, they have 70S ribosome that has one small subunit (30S) and a large subunit (50S). Their genome is haploid with a single chromosome that is circular and a short-lived unprocessed mRNA. Special information may be contained in transposons and replicons.
Prokaryotic microorganisms do not have sexual reproduction. They transmit their genetic material through three ways. These include: transformation, conjugation, and transduction. In transformation, the cell takes up the genetic material from the surrounding environment. In conjugation, there is direct transfer of genetic components from one prokaryotic organism to the other using pilus. Lastly, transduction is the other means of reproduction. It occurs when viruses transfer genetic material between prokaryotes.
Prokaryotes move by fimbrae, pilus, or flagella. They have four types of flagellation, monotrichous (single flagella at one end), lophotrichous (multiple flagella at one end), amphittrichous (flagella located at each end) and peritrichous (flagella located around the surface).Pilus is a long tubular structure that is used for movement, conjugation and attachment. These cells move through chemotaxis, which is movement of the cell in response to chemical stimuli. The stimuli can be positive (towards) or negative (away).
Prokaryotes like some Gram-positive bacteria survive in harsh environmental conditions like excessive heat and dessication. They do this by generating endosporic forms that exist as spores. Vegetative form is the metabolically active stage of spores while endospore is the dormant stage of spores but capable of becoming metabolically active (vegetative cell). Most endospore forming bacteria are less harmful to humans, for example, the nitrogen – fixing bacteria. However, others are harmful and may cause diseases like salmonella, diphtheria, and bubonic plague. Most prokaryotic cells produce sticky substances that enable them to adhere to food substrate as well as form a protective capsule. These organisms grow and adapt quickly in the environment in which they are found. Most of prokaryotic microorganisms cause disease by producing poisonous substances that can be exotoxins, that is, substance secreted by the pathogenic microorganism or endotoxins, that is, part of the pathogenic microorganism’s morphology.
In summary, prokaryotic cells are unicellular microorganisms which are found almost everywhere in the world. They are virtually smaller than eukaryotes. They occur in various shapes and have asexual means of production. In addition, they have two types of cell walls that are grouped according to how they stain. Thus, there are Gram positive and Gram-negative bacteria. Most of them are beneficial or less harmful but some remain to be harmful to humans. Lastly, prokaryotes survive harsh environmental conditions by existing as endospores or secreting toxic substances that neutralize harmful substances. In addition, prokaryotes have been used in research and technological innovations like treatment of sewage, bioremediation, medical production, and genetic engineering among others.
Chapter 23: Parasites of medical importance
This chapter covers Protozoa and helminthes that are of clinical significance. It looks at structure, epidemiology, pathogenic mechanism and lifecycles as well as means of control of protozoa and helminthes. It also describes some of the diseases and conditions that occur as a result of parasitic infestation.
Protozoa are unicellular organisms and helthminths comprises worms. Protozoan parasites occur in two distinct forms. Trophozoite, is the motile form and cyst. Additionally, it is the dormant or resting stage, which is the form in which it is transmitted from one host to the other. They have both sexual and asexual phases of production. Protozoan parasites can be classified into Amoeboid protozoa which move by use of pseudopods. Secondly, there are Flagellates that move by flagella. Thirdly there are Ciliated protozoa, which move by means of cilia. Fourthly, there are Apicomplexa which have an apical complex that help them be taken up by the cells of the host.
There are many protozoans of medical significance discussed in this chapter. Intestinal and urogenital protozoans of medical essence include: Entamoeba histolytica, Giardia lamblia, Cryptosporidium sp., Balantidium coli, and Trichomonas vaginalis. Blood and tissue protozoa include Plasmodium, Leishmania, Trypanosoma, Pneumocystis and Babesia species.
Cryptosporidium sp. is a protozoa that is common among Acquired Immunodeficiency Syndrome (AIDS) patients and is among the microorganisms that cause severe diarrhea in such patients. Moreover, these microorganisms affect individuals with compromised immunity and especially those who have been exposed to contaminated water. This is because Cryptosporidium sp. demonstrates resistance to chlorination. The microorganism is also transmitted to humans through undercooked meat, which has cysts.
Entamoeba histolytica ingests white blood cells and red blood cells. These microorganisms are transmitted to human beings who are the primary hosts when they consume water, fresh fruits, and vegetables that have been contaminated with faecal matter that has cysts. Affected human beings then develop diseases like amoebiasis, dysentery, and intestinal abscess. Amoeba secretes enzymatic substances that dissolve tissues and penetrate into the deeper layers of the intestinal mucosa causing dysentery, abdominal pain, fever, and weight loss. However, infection with this organism is usually asymptomatic in about 90% of patients. It may also invade the lungs, liver, and the brain.
Giardia lamblia is a protozoan associated with giardiasis, which is diarrhea that occurs with malabsorption. It is transmitted through the faecal oral route or through anal intercourse with someone with the cysts. This protozoa is distributed globally but more common in the tropical and subtropical regions.
Plasmodium parasites have two different hosts and the human being or cattle act as the intermediate host in the asexual stage of its life cycle. These species are responsible for causing malaria in humans. Plasmodium is responsible for causing malaria in humans and exists in four species that include P. ovale, P. vivax, P. malariae and P. falciparum. P. falcipaparum is the commonest in the tropics and subtropics. However, P.ovale is rare. Treatment of such infection is by use of chloroquine, quinine or pysimethamine- sulfadoxine.
Balantidium coli are a protozoan that is associated with dysentery, and colitis. It is transmitted through food or water. B.coli is distributed worldwide and risk factors for human infection include contact with swine and unhygienic conditions.
Trichomonas vaginalis is a protozoan parasite transmitted to humans through sexual intercourse. It causes trichomoniasis in humans. The disease it causes presents as vaginal and/or urethral discharge and itchiness. This parasite is found all over the world and sexual contact is the primary means of acquisition. It is more common in women than men.
Babesia microfti is a parasite that causes malaria like symptoms (babesiosis) in humans. Infection with these parasitic organisms is also associated with chills, fever, anemia, and general body weakness. It is transmitted to humans by ticks.
Trypanosomes and Leishmania sp are protozoan parasites that infect human blood and tissues like the cardiac tissue causing leishmaniasis. Leishmaniasis can occur in different forms that include visceral, cutaneous and mucocuataneous leishmaniasis. Leishmania donovani causes visceral leishmaniasis while T. cruzi causes Chagas’ disease. This is an acute disease that occurs in young children and may spread to the brain. Leishmania Mexicana complex causes cutaneous leishmaniasis and Leishmanis braziliensis complex causes muco-cutaneous leishmaniasis. These conditions are common in South and Central America. Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense cause African sleeping sickness. Most of these protozoan infections are treated with drugs like iodoquinol and metronidazole among other medications .
This chapter also discusses helminthes parasites that are of medical significance. Helminthes parasites exist as worms in humans. Adult worms mate and produce fertilized eggs that hatch to produce larvae. Larvae then go through several stages after which they mature into adults. Adults exist in definitive host while eggs and larvae develop in intermediate host.
Pathology from helminthes develops from worms feeding on and migrating through tissues and secretion of worm products of metabolism. Diagnosis of infection with helminthes is usually made from white blood cell count, serological tests, blood and tissue biopsies, and visualization of eggs, larvae, and adult worms in faeces, sputum, and urine.
Helminthes of medical significance include nematodes, also referred to as roundworms, trematodes, and cestodes. Roundworms usually inhabit the intestines. There are different types of nematodes that are transmitted to humans through the faecal-oral route. Those infective in embryo stage are Trichuris trichiura, Ascaris lumbricoides and Enterobius vermicularis. Those that infect humans in larval stage include Necator Americanus, Ancylostoma duodenale, Trichinella spiralis,and Strongyloides stercoralis.
Tissue nematodes include Wuchereris bancrofti (filariasis), Onchocerca volvulus (river blindness), and Loa loa (eye worm). Filariasis is spread by a mosquito bite. On the other hand, river blindness is spread by a bite by black fly. Eye worm is spread by a bite of mangrove or deer flies. On the other hand, trematodes include S. japonicum, S. mansoni, S, haematobium and F. hepatica. They are all blood flukes transmitted to humans by snails. Humans are infected when they consume fresh water, fish, and other seafood that has larvae of these organisms.
The last group of helminthes is cestodes, which comprises of Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm) D. latum (fish tapeworm), and H. nana. They are transmitted to humans when they consume undercooked or raw beef, pork, or fish. Infestation by Taenia solium can cause seizures, damage to the myocardium, eyes, brain and even lead to psychiatric disturbances.
Infestation by helminthes like Ascaris lumbricoides can lead to allergic and inflammatory reactions. Heavy infestation can lead to anemia due to internal blood loss, retarded physical and mental development due to loss of nutrients, dysentery and even death especially in young children.
Heavy infestation by whipworm, Trichuris trichiura can lead to rectal prolapsed, and diarrhea. Hookworms may cause pneumonia, abdominal pain and gut related complications including blood stained diarrhea. In addition, pneumonitis, enlargement of the liver, intestinal obstruction and malabsorption syndrome can result. Helminthes of medical importance can also transmit pathogenic organisms to humans in the process of moving from one region to another or feeding.
Parasites that include protozoa and helminthes have myriad effects on the host. Some of the negative effects include mechanical injury to cells and tissues of the host by the parasite. This mechanical damage can be caused by pressure as the parasite increases in size or as it moves from one point to the other. This may eventually lead to blockage of ducts and other vessels. Parasites deprive the host of nutrients, and fluids since they compete with him/her for the same. Additionally, parasites secret toxic substances. These substances are released into the body system of the host. They cause illness and immunologic reactions. However, there are positive effects or significance of parasites. Some protozoa are used in biological treatment of sewage through their anaerobic digestion. Thus, this increases the safety level of sewage once it is discharged into rivers or sewage lines. Parasites are also used in medical research programs for study purposes or invention of drugs. Parasites also provide an avenue for career opportunities like parasitology. Thus, it should not always be assumed that parasites are harmful or have negative effects on humans.
Talaro, Kathleen P, K Cowan Marjorie and Chess Barry. Foundations in Microbiology: Basic principles. Boston: McGraw Hill Higher Education, 2009.