ASCPi-MICROBIOLOGY

Antimicrobial susceptibility testing (AST) is performed on bacteria isolated from clinical specimens to determine which antimicrobial agents might be effective in treating infections caused by the bacteria. Only bacteria that are likely to be contributing to an infection should be tested.

Susceptibility testing is usually performed by a disk diffusion or dilution (minimal inhibitory concentration) method.

Mycoses (singular, mycosis) are diseases caused by fungi. Fungal disease is categorized based on the site of the infection—superficial, cutaneous, subcutaneous, and systemic mycoses.

E. histolytica and E. dispar are morphologically identical organisms that differ in their effects on the host. Recently, Entamoeba moshkovkii, has been identified as belonging to this complex. E. dispar is considered a nonpathogenic E. histolytica, whereas the pathogenic organism remained E. histolytic and E. moshkovkii is thought to be a freeliving organism

Free-living, thermotolerant amebae found in soil and water usually feed on bacteria in the environment but are known to be capable of causing human disease. They are able to gain access to the CNS by inhalation into the upper respiratory tract followed by penetration of the nasal mucosa, which allows them to travel along the olfactory nerve to the brain, or by hematogenous spread from the lungs or skin lesions. These infections are very difficult to diagnose and treat and are associated with a high mortality rate.

Plasmodium spp., which cause malaria, remain endemic throughout the world in tropical and subtropical countries. Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparum are the causative agents of human malaria. In general, infections caused by P. vivax, P. ovale, and P. malariae are less severe than those caused by P. falciparum. A fifth species, Plasmodium knowlesi, which causes malaria in the macaque monkey, has also been implicated in cases of zoonotic human malaria in Malaysia and the Philippines.

Babesiosis is a zoonotic intraerythrocytic infection transmitted by the bite of a tick. Although Babesia microti and B. Babesia hominis, have been reported to infect humans, B. microti is the most common. The hard tick (Ixodes spp.) serves as the vector, and white-footed mice and white-tailed deer are the reservoirs. There also have been simultaneous transmission of B. microti and Borrelia burgdorferi (the causative agent of Lyme disease) because these two organisms use the same tick vector.

Helminth infections in humans are caused by flukes, tapeworms, or roundworms. Humans become infected by directly ingesting eggs (ova), by ingesting larvae in an intermediate host, or through direct larval penetration of the skin. The pathology and severity of infection are related to the number of adults present, referred to as the worm burden. Most of the parasites inhabit the intestinal tract, but species also infect the liver, lungs, lymphatics, and blood vessels.

Human roundworms include those that infect the intestinal tract and blood and tissue. These organisms are transmitted by the ingestion of the embryonated egg or by direct penetration of the skin by larvae in the soil, or they may require an insect vector. Roundworms are characterized by the presence of two sexes and a life cycle that may involve larval migration throughout the body. The adults obtain nourishment by absorbing nutrients from partly digested intestinal contents or by sucking blood. Patients may be asymptomatic or symptomatic; the severity of the symptoms is related to the worm burden, host’s nutritional status and age, and duration of infection. Most roundworm infections can be treated with oral albendazole or mebendazole.

Formalin can be used when a wet mount or concentration procedure (sedimentation or flotation) will be performed. The fixative sodium acetate–acetic acid–formalin (SAF) can be used for the preservation of fecal specimens when concentration procedures and permanent stains will be used. Vials of merthiolate-iodine-formalin (MIF) can also be used to preserve trophozoites, cysts, larvae, and helminth eggs for wet mount or concentration procedures. This preservative, however, is not routinely used for permanently stained smears.