Cell Wall Defective Bacteria

Mycoplasmas and Cell Wall Defective Bacteria

Mycoplasmas
smallest known free-living organisms

Due to their smallest size, originally thought to be viruses, but differ in

i) Contain both RNA and DNA

ii) Can synthesize proteins by their own enzymes
iii) Can multiply on their own

Classification

For many years thought to be viruses
Nowdays classified as atypical bacteria because they lack the cell wall
At least 15 spp are thouhgt to be of human origin, only 4 spp are of medical importance

Spp.

Disease
M.pneumoniae
. atypical pnuemoina
M.hominis
. postpartum fever
. pelvic inflamatory diseases
Ureaplasma urealyticum
. non gonococcal urethritis
. lung diseases in premature infants of low birth weight (LBW)
M.genitalium
. Urethral infections


General characterisitcs
• Smallest organisms that can be free living 125-250 nm in size
• Highly pleumorphic because of lack of rigid cell wall
• Completely resistant to penicillin and cephalosporins because of lack of cell wall
• Can reproduce in cell free media, on agar produce colonies of fried egg appearance
• Have affinity for mammalian cell membrane

Morphology

Stain poorly with bacteria stain (gram stain), but stain well with Giemsa stain

They lack cell wall so:

Highly pleomprphic
Not stained with gram stain
Can’t be killed by penicillin

On Giemsa stain they appear as tiny pleomorphic cocci, short rods, short spirals, and sometimes as hollow ring forms. Their diameter ranges from 0.15 µ to 0.30 µ






Growth and Cultural Characteristics
Cannot be studied by usual bacteriologic methods because of the small size of their colonies and delicacy due to lack of rigid cell wall

Facultative anaerobes except for M.pneumoniae that are strict aerobe

Grows better on H2 and N2 with 10% CO2

Most grow at 370C. Generation time slow, 1-6 hrs

Use glucose as energy source

Many strains grow in heart infusion peptone broth with 2% agar, pH 7.8 and animal serum or 30% ascitic fluid added

Mycoplsama contain sterols in their cell membrane, so their growth requires the addition of serum or cholestrol to the medium to produce sterols, Ureaplasma reqiure urea

After 2-6 days of incubation, produce small colonies that have freid- egg appearance

Pathogenesis
Pathogenic Mycoplasma have polar tips structures-mediate adherence to host cells

Direct cytotoxicity through generation of hydrogen peroxide and superoxide radicals

Cell lysis-by antigen antibody reactions/ chemotaxis and action of mononuclear cellls

Medical importance

Part of normal flora of mouth, genitourinary tract especially females

i) Asymptomatic

Many infections due to Mycoplsmas are asymptomatic.

Medical importance

ii) Mycopasma pneumoniae and atypical pneumonia (walking pneumonia)

M.pneumoniae is a prominant cause of a typical pneumonia especially in persons 5-20 years of age
transmitted by infected respiratory secretions
Incubation period 1-3 wks
Generally mild disease, but can range from asypmtomatic infection-serious pneumonitis
Complications-neorologic, hemolytic anaemia, skin
lesions


Medical importance
iii) Mycoplsma hominis

uterine tube infections (salpingitis and tubo-ovarian abscesses) 10%

post abortal or post partum fever(10%)

Occasionally arthritis

Medical importance
IV) Ureaplasma urealyticum

Common in female genital tract

Attributed to some cases of non gonococcal urethritis in men

Associated with lung disease in premature LBW infants

Weak evidence correlates U.urealyticum to infertility

Medical importance

V) Mycoplasma genitalium


Attributed to some cases of non acute and chronic non-gonococcal urethritis in men

In females, M.genitalium has been associated with cervicitis, endometritis salpingitis and infertility

Diagnosis

Specimen include,
blood,sputum,nasopharyngial or urethral exudates

Direct staining is of limited use. For direct detection in clinical specimen :

. IF
. PCR

Culture is difficult and time consuming

Diagnosis
Serology (most widely used)
Detection of IgM or rising titer of IgG by ELISA or
CFT (acute and convalescent phase sera are
necessary to demonstrate a fourfold rise in the CF
antibodies)


Cold agglutinins at a titer of 1/128 or higher =
acute infection (Heamoaglutination)

Certain antigens on human red blood cells are identical to Mycplasma pneumonia antigens. So, antibodies to mycoplasma cross react with human red blood cells causing them to agglutinate at 4 degree but not at 37 degree

Positive in 50 % of cases

Not specific. Positive in other diseases like viral infections & malaria

Cold agglutinins?

Positive in?
Specific?

Treatment

Lack of cell wall make organisms resitant to antimicrobials acting on cell wall e.g penicillines, cephalosporins and vancomycin

Sensitive to antimicrobials that inhibit protein synthesis e.g. tetracyline, erythromicin , azithromycin and chloramphenical

Other Cell wall- defective bacteria(L-forms bacteria)

• First isolated in 1935 by Emmy Klienburger who named them L-forms (Lister Institute in London where she was working)

Don’t occur naturaly and not genetically related to mycoplasma


Usually result from
• Spontaneous mutation
Effects of chemicals
Enzymes (lysozymes)
Antibiotic treatment

Other Cell wall- defective bacteria(L-forms bacteria)

Two types:

Protoplast (stable L-forms) usually derived from gram positive bacteria in which the cell wall is completely destroyed.They can’t be reverted to their original morphology.

Spheroplast (unstable L-forms) usually derived from gram negative bacteria in which cell wall is not completely destroyed; they retain some outer membrane material and can revert to their original morphology.

Their role in disease is uncertain; may be responsible for the recurance of the infection after antimicrobial treatment.

L Forms vs Mycoplasma

contain a rigid cell wall, at least at one stage of their life cycle

no sterols in their cytoplasmic membrane