Chlamydiae, Rickettsiae, Mycoplasmas

These microbes are grouped together because, like other bacteria, they divide by binary fission and are sensitive to antibiotics, but they lack certain structures (e.g., Mycoplasma lack a cell

wall) or metabolic capabilities (e.g., Chlamydia cannot synthesize adenosine triphosphate [ATP]). Chlamydia and Rickettsiae are obligate intracellular organisms that replicate in

membrane-bound vacuoles in epithelial cells and the

Figure 8-2Molecules on the surface of Gram-negative and Gram-positive bacteria involved in pathogenesis. Not shown is the type 3 secretory apparatus of Gram-negative bacteria (see

text).

Figure 8-3The variety of bacterial morphology. A, Gram stain of sputum from patient with pneumonia. There are Gram-positive cocci in clusters (Staphylococcus aureus) with

degenerating neutrophils. B, Gram stain of sputum from a patient with pneumonia. Gram-positive, elongated cocci in pairs and short chains (Streptococcus pneumoniae) and a neutrophil is

seen. C, Gram stain of Clostridium sordellii grown in culture. A mixture of Gram-positive and Gram-negative rods, many of which have subterminal spores (clear areas), are present.

Clostridia species often stain as both Gram-positive and negative, although they are true Gram-positive bacteria. D, Gram stain of a bronchoalveolar lavage specimen showing Gramnegative

intracellular rods typical of Enterobacteriaceae such as Klebsiella pneumoniae or Escherichia coli. E, Gram stain of urethral discharge from a patient with gonorrhea. Many Gramnegative

diplococci (Neisseria gonorrhoeae) are present within a neutrophil. F, Silver stain of brain tissue from a patient with Lyme disease meningoencephalitis. Two helical spirochetes

(Borrelia burgdorferi) are indicated by arrows. The panels are at different magnifications. (D, Courtesy of Dr. Karen Krisher, Clinical Microbiology Institute, Wilsonville, OR. All other

panels courtesy of Dr. Kenneth Van Horn.)

TABLE 8-6-- Protozoa Pathogenic for Humans

Species Order Form, Size Disease

Luminal or Epithelial

Entamoeba histolytica Amebae Trophozoite 15–20 μm Amebic dysentery; liver abscess

Balantidium coli Ciliates Trophozoite 50–100 μm Colitis

Naegleria fowleri Ameboflagellates Trophozoite 10–20 μm Meningoencephalitis

Acanthamoeba sp. Ameboflagellates Trophozoite 15–30 μm Meningoencephalitis or ophthalmitis

Giardia lamblia Mastigophora Trophozoite 11–18 μm Diarrheal disease, malabsorption

Isospora belli Coccidia Oocyst 10–20 μm Chronic enterocolitis or malabsorption or both

Cryptosporidium sp. Coccidia Oocyst 5–6 μm

Trichomonas vaginalis Mastigophora Trophozoite 10–30 μm Urethritis, vaginitis

Bloodstream

Plasmodium species Hemosporidia Trophozoites, schizonts, gametes (all small and

inside red cells)

Malaria

Babesia microti, B. bovis Hemosporidia Trophozoites inside red cells Babesiosis

Trypanosoma species Hemoflagellates Trypomastigote 14–33 μm African sleeping sickness

Intracellular

Trypanosoma cruzi Hemoflagellates Trypomastigote 20 μm Chagas disease

Leishmania donovani Hemoflagellates Amastigote 2 μm Kala-azar

Leishmania species Hemoflagellates Amastigote 2 μm Cutaneous and mucocutaneous leishmaniasis

Toxoplasma gondii Coccidia Tachyzoite 4–6 μm (cyst larger) Toxoplasmosis

immunosuppressed individuals do opportunistic fungi give rise to life-threatening infections characterized by tissue necrosis, hemorrhage, and vascular occlusion, with minimal to no

inflammatory response. In addition, AIDS patients are victims of the opportunistic fungus Pneumocystis jiroveci (carinii).

Protozoa

Parasitic protozoa are single-celled eukaryotes that are major causes of disease and death in developing countries ( Table 8-6 ). Protozoa can replicate intracellularly within a variety of

cells (e.g., Plasmodium in red blood cells, Leishmania in macrophages) or extracellularly in the urogenital system, intestine, or blood. Trichomonas vaginalis are flagellated protozoal

parasites that are sexually transmitted and can colonize the vagina and male urethra. The most prevalent intestinal protozoans, Entamoeba histolytica and Giardia lamblia, have two forms:

(1) motile trophozoites that attach to the intestinal epithelial wall and may invade and (2) immobile cysts that are resistant to stomach acids and are infectious when ingested. Blood-borne

protozoa (e.g., Plasmodium, Trypanosoma, and Leishmania) are transmitted by insect vectors, in which they replicate before being passed to new human hosts. Toxoplasma gondii is

acquired either by contact with oocyst-shedding kittens or by eating cyst-ridden, undercooked meat.

Helminths

Parasitic worms are highly differentiated multicellular organisms. Their life cycles are complex; most alternate between sexual reproduction in the definitive host and asexual

multiplication in an intermediary host or vector. Thus, depending on parasite species, humans may harbor either adult worms (e.g., Ascarus lumbricoides) or immature stages (e.g.,

Toxocara canis) or asexual larval forms (e.g., Echinococcus species). Once adult worms take up residence in humans,

they do not multiply but generate eggs or larvae destined for the next phase of the cycle. An exception is Strongyloides stercoralis, the larvae of which can become infectious in the gut and

cause overwhelming autoinfection in immunosuppressed persons. There are two important consequences of the lack of replication of adult worms: (1) Disease is often caused by

inflammatory responses to the eggs or larvae rather than to the adults (e.g., schistosomiasis), and (2) disease is in proportion to the number of organisms that have infected the individual (e.

g., 10 hookworms cause little disease, whereas 1000 hookworms cause severe anemia by consuming 100 mL of blood per day).

Ectoparasites

Ectoparasites are insects (lice, bedbugs, fleas) or arachnids (mites, ticks, spiders) that attach to and live on or in the skin. Arthropods may produce disease directly by damaging the human

host or indirectly by serving as the vectors for transmission of an infectious agent into a human host. Some arthropods may cause itching and excoriations (e.g., pediculosis caused by lice

attached to hair shafts, or scabies caused by mites burrowing into the stratum corneum). At the site of the bite, mouthparts may be found associated with a mixed infiltrate of lymphocytes,

macrophages, and eosinophils. In addition, attached arthropods can be vectors for other pathogens. For example, deer ticks transmit the Lyme disease spirochete Borrelia burgdorferi.

Date: 2016-04-22; view: 1026