Rapid Growers
Group IV, or rapid growers, is a heterogenous group of myco-bacteria that produce visible growth on LJ medium rapidly within 1 week of incubation at 37°C or 25°C. This group may also include photochromogens, scotochromogens, or nonpho-tochromogens species.
Some of the rapid growers are chromogenic (Mycobacteriumsmegmatis, Mycobacterium phlei) and are saprophytes. These rapid-growing mycobacteria include five species which are recog-nized as important opportunistic pathogens of humans. These Mycobacteriumspecies are M. fortuitum, M. chelonae, M. abscessus, M. smegmatis, and M. phlei. All these rapid growers:
· stain irregularly with acid-fast stains;
· show relatively low virulence; and
· are relatively more susceptible to treatment with conven-tional antibacterial antibiotics than to antimycobacterial agents.
These bacteria are usually associated with diseases of deep sub-cutaneous tissues following trauma or iatrogenic infections. These species have been isolated from contaminated wound dressing, infections associated with intravenous catheter, or infections associated with prosthetic devices, such as heart– lung machine. Such infections are now being increasingly recognized from hospitalized patients due to increase in the use of such medical devices.
M. fortuitum and M. chelonae are nonchromogens; they donot produce any pigments. Both the species cause chronic abscesses in humans following infections of vaccines and other preparations contaminated by these mycobacteria. The infec-tion also occurs following some injury. These bacilli are usually found in the soil. M. fortuitum causes pulmonary disease similar to pulmonary tuberculosis. Both M. chelonae and M. fortuitum grow well at 35°C and form white- to cream-colored colonies and reveal coccoid to filamentous morphology on staining.
phlei is mostly nonpathogenic. It produces buff-coloredcolonies, which on further incubation become yellow to orange. M. phlei differs from M. smegmatis by its ability to grow at 52°Cand survive heating at 60°C for 4 hours.
smegmatis is normally found in smegma, a whitish secre-tion around the orifice of urethra. These bacilli are slender,curved, and rigid rods. These bacilli are both acid and alcohol fast, but some bacilli may only be acid fast but not alcohol fast. M. smegmatis is a rare human pathogen associated with occa-sional cases of lung, skin, and bone infections.
Differential properties of important atypical Mycobacterium species are summarized in Table 42-2.
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