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NONFERMENTERS
The nonfermenters are classified as such because of the way they
metabolize glucose and other carbohydtrates. Specifically, the bacteria
discussed in this section are Gram-negative rods that either don't ferment
glucose for energy or do not use glucose at all. This is a very diverse group of
organisms in that they are not classified by DNA homology. These are also some
of the most difficult bacteria to identify because they are negative for so many
tests. Many nonfermenters do not grow on MacConkey agar. The two most common
identification used for these organisms are the OF-glucose test and the triple
sugar iron (TSI) test. We will discuss only the clinically significant
organisms.
PSEUDOMONAS
ACINETOBACTER
OTHERS
PSEUDOMONAS
Pseudomonads are motile, Gram-negative rods that utilize glucose
oxidatively. Members of this genus are classified into five groups based on
ribosomal RNA homology. These bacteria are clinically important because they are
resistant to most antibiotics and they are capable of surviving in conditions
that few other organisms can tolerate. They also produce a slime layer that is
resistant to phagocytosis. Pseudomonas is often encountered in hospital
and clinical work because it is a major cause of hospital acquired (nosocomal)
infections. Its main targets are immunocompromised individuals, burn victims,
and individuals on respirators or with indwelling catheters. Additionally, these
pathogens colonize the lungs of cystic fibrosis patients, increasing the
mortality rate of individuals with the disease. Infection can occur at many
sites and can lead to urinary tract infections, sepsis, pneumonia, pharyngitis,
and a lot of other problems. Rarely will you find Pseudomonas as a cause
of infection in healthy individuals. Its non-invasive nature limits its
pathogenic capabilities.
P. aeruginosa
Pseudomonas aeruginosa is the most frequently isolated non-fermenter
in the laboratory. It has several features that distinguish it from other
species of Pseudomonas:
- It can grow at 42 degrees celsius
- Produces a bluish pigment (pyocyanin) and a greenish pigment
- Characteristic fruity odor
The basis of this organisms
pathogenicity involves several toxins and chemicals which the bacterium secretes
upon infection. The lipopolysaccharide layer helps the cell adhere to host
tissues and prevents leukocytes from ingesting and lysing the organism. Lipases
and exotoxins then procede to destroy host cell tissue which then leads to the
complications associated with infection. P. aeruginosa prefers to inhabit
moist environments but it can survive in a medium as deficient as distilled
water. It will also grow on just about any laboratory medium and is
beta-hemolytic on blood agar. Treatment of Pseudomonas infection consists
of a combination of two antibiotics: for example an anti-pseudomonal penicillin
and an aminoglycoside. The best way to reduce the spread of P. aeruginosa
in the hospital is to use good aseptic technique on hospital instruments and
when in contact with patients.
LABORATORY INDICATIONS:
- Oxidase +
- Beta-hemolytic
- Characteristic odor and color
- Motile
OTHER SPECIES
Burkholderia (Pseudomonas) cepacia is an opportunistic pathogen of
cystic fibrosis patients. It also shows substantially greater antibiotic
resistance than its relative, P. aeruginosa. However, B. cepacia
can be distinguished from Pseudomonas species because it is lysine
positive.
Although not of the same genus, Stenotrophomonas maltophila (formerly
known as Xanthomonas maltophila) is very similar to the Pseudomonads.
This motile bacterium is a cause of nosocomal infections in immunocompromised
patients. S. maltophila also harbors significant resistance to many
antibiotics considered effective for treating Pseudomonas infections.
However, most strains of the bacterium are susceptible to trimethoprim
sulfamethoxazole. S. maltophila can also be distinguished from
Pseudomonas species by the lysine and DNAse tests for which it is
positive.
ACINETOBACTER
Acinetobacter species are oxidase-negative, non-motile bacteria which
appear as Gram-negative coccobacilli in pairs under the microscope. Identifying
the different species of this genus can be done through the use of FLN
(Flourescence-Lactose-Denitrification medium) acid results which determines the
amount of acid produced from metabolizing glucose. Also, most members of
Acinetobacter show good growth on MacConkey agar with the exception of
some A. lwoffii strains. Although many species of Acinetobacter
can cause infection, A. baumannii is the most frequently encountered
species in the clinical laboratory. Like Pseudomonas, A. baumannii
can be linked to many hospital acquired infections including skin and wound
infections, pneumonia, and meningitis. A. lwoffi, in particular, is
responsible for most cases of meningitis caused by Acinetobacter. Because
most species are resistant to penicillin and chloramphenicol, a combination of
aminoglycoside and ticarcillin is usually recommended for treatment.
LABORATORY INDICATIONS:
- Oxidase -
- Non-motile
- Penicillin resistance (most strains)
OTHER NONFERMENTERS
Flavobacterium
Members of Flavobacterium are ubiquitous organisms that can cause
infection in premature infants and immunocompromised individuals. They are not
difficult to distinguish from other nonfermenters because most species produce
indole when grown in tryptophan broth. While most species of
Flavobacterium metabolize glucose oxidatively, some have been shown to be
really slow fermenters. In any case, all species are motile and oxidase
positive. The species most often recovered from humans is F.
meningosepticum, a penicillin resistant bacterium that can cause neonatal
meningitis. Vancomycin is usually effective in Flavobacterium infection.
Alcaligenes
All the species in this genus are oxidase positive, motile, and can grow on
MacConkey agar. The most coommon species A. faecalis, an opportunistic
pathogen which usually inhabits soil and water. This organism has a
characteristic fruity odor which distinguishes it from other Alcaligenes
species.
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