Neisseria gonorrhoeae (“The Gonococcus”)– Gonorrhea
Reading Assignments: (1) Text Chapter 14, 66 (2) Plates 95, 96, and 98, In Kapit, W. and L.M. Elson. 1977. The Anatomy Coloring Book, Harper and Row Publishers, New York. (3) Gilligan, P.H., M.L. Smiley, and D.S. Shapiro. 1997. (Case 2, pp.9-12) In: Cases in Medical Microbiology and Infectious Diseases (2nd ed.), American Society for Microbiology, Washington, D.C., (6) Cunha, B.A., 1999. (Patients 2 and 6) Infectious Disease Pearls, Hanley and Belfus, Inc., Philadelphia.
A. 1st description by Hippocrates (460-355 BC)- dissected inflamed urethra
B. Gonorrhea – (from Gr. gone, seed + Gr. rhein, to flow) - Galen (120-200 AD).
C. Neisseria gonorrhoeae – Neiser – 1879 – demonstrated inside PMN's from pus taken from 35
cases of gonorrhea but not from simple vaginal discharges; demonstrated as the cause of
ophthalmia neonatorum.
D. Bumm – 1885 – cultivated the organisms, inoculated human subjects and proved Koch's
postulates
E. Today – one of the most prevalent sexually transmitted diseases
2. Microbiology
Gram negative cocci that typically appear in pairs (diplococci with flattened opposing
sides). Structure is typical of a Gram negative cell – inner cytoplasmic membrane, a thin
peptidoglycan layer, and an outer membrane containing phospholipids, proteins and
lipooligosaccharide (LOS) rather than LPS.
3. Epidemiology
A. N. gonorrhoeae is an obligate human pathogen
B. Major reservoir – the asymptomatic carrier. (People with symptoms will seek medical help
and generally stop sexual activities. About 30% of women and 10% of men are asymptomatic.
Unless a sexual contact reports a symptomatic infection, these infected individuals will
continue to spread the disease
C. Transmission
1. Sexual contact (women have a 50% risk of infection after a single exposure to an
infected man; men have a 20% risk as the result of exposure to a single infected
woman. The risk of infection increases as the number of sexual encounters with an
infected partner increases.
2. Transmission from an infected mother to her child at the time of birth.
D. Individuals most at risk
1. Patients with multiple sexual encounters
2. Patients with deficiencies in the late complement components (DGI)
4. Clinical Syndromes (See Text Table 14.1, p. 163) (See Reading Assignment (2) for a basic
understanding of the anatomy involved and review the nonspecific defenses of the genitourinary tract
presented in Lecture 35).
A. Lower and Upper Reproductive Tract Infections – infections occur in the columnar (not
squamous) epithelial cells of males and females (males – distal urethra, females- cells of
the cervix, not the vagina)
IN THE MALE:
1. Urethritis of the anterior urethra (lower genital tract infection)
a. Incubation period of 2-5 days, followed by symptoms of a purulent urethral
discharge and dysuria (difficulty or pain on urination). Approx. 95% of all
infected men have acute symptoms. (See Case 2 in Reading Assignment (3)
and Patient 2 in Reading Assignment (4)
2. Complications are rare but may include upper reproductive tract infections:
a. Epididymititis- inflammation of the epididymis
(Epididymis – the coiled duct which connects the testes with the vas
deferens. Spermatozoa, which are made in the testes, are stored and
mature in the epididymis before passing into the vas deferens.)
b. Prostatitis – inflammation of the prostate
IN THE FEMALE
1. Cervicitis
a. Primary site in the adult female is the cervix because the N. gonorrheoea
cannot colonize the mature squamous epithelial cells of the vagina.
The glycogen-rich mucosa is an inhospitable environment for the growth of
the pathogen. Lactic acid produced by the resident lactobacilli from glycogen
in the squamous epithelium protects the adult vagina against gonococci.
Note: In prepubescent girls, the vagina and vulva are lined with
immature squamous epithilium which lacks glycogen and therefore is not
protected in the same way. Thus, sexually abused girls may present with
vulvovaginitis rather than cervicitis.
b. Symptoms – vaginal discharge, dysuria, and abdominal pain (easily mistaken
for a UTI - cystitis)
c. Remember – many of these infections are asymptomatic, but can later result
in serious complications!
2. Ascending Upper Reproductive Tract Infections:
a. Pelvic inflammatory disease (PID) – an ascending infection of the upper
reproductive tract including the uterus, fallopian tubes, ovaries, and adjacent
peritoneal linings. (See Text p. 616-617; Case Study Ms. P., p. 162)
The host defenses that work against the ascent of the organisms –
include tubal ciliary movement, flow of mucus towards the uterus and
myometrial contractions during menses – that result in sloughing of the
endometrium.
(1) Endometritis – inflammation of the uterus
(2) Salpingitis – inflammation of the fallopian tubes
b. Pathogenesis of PID
1. Usually follows a primary gonococcal infection.
2. Influenced by hormonal factors – most cases occur within 7 days of
the beginning of menses- (1) hormonal changes may change the
cervical mucus plug and allow passage of organisms, (2) the reflux of
blood during menstrual uterine contractions may allow entry into the
fallopian tubes.
3. Infection of the cervix is followed by passage of gonococci across
the endocervical mucus plug (damaged by the infection, or changed
as the result of hormonal influences) and ascension of the pathogen
into the uterus and fallopian tubes.
4. The gonococci multiply at these sites and the host's nonspecific
inflammatory response to the infection damages the tissues resulting
in fibrosis and scarring.
The results:
1. Chronic abdominal pain
2. Scarring of the fallopian tubes and infertility
3. Ectopic pregnancy – the egg becomes trapped in the
fallopian tube, is fertilized and the fetus begins to
develop there. If the fetus is not detected and
surgically removed, the tube can rupture, causing the
woman to go into shock.
B. Nonreproductive Tract Infections:
1. Proctitis (rectal gonorrhea) – inflammation of the rectum
2. Pharyngitis
3. Ophthalmia neonatorum – contamination of the eyes during birth causes bilateral
conjunctivitis; if untreated, leads to permanent damage to the eyes and blindness.
Readily prevented by administering antibiotic ointment or drops into the eyes
immediately after birth. (Originally prevented by instilling 1% silver nitrate into the
eyes.
4. Extension of infection to areas contiguous with the pelvis causing peritonitis or
perihepatitis (Fitz-Hugh-Curtis Syndrome) – inflammation of the peritoneal capsule of
the liver and of the tissues around the liver.
C. Disseminated Gonococcal Infection (DGI) – occurs in 1-3% of those with gonorrhea. (See
Patient 6 - Reading Assignment (4). More common in women than in men. Overt signs of
genital disease may be absent.
1. Gonoccoci disseminate to the skin and joints
2. Symptoms
a. Arthritis-dermatitis syndrome – fever, flu-like symptoms and flitting
polyarthralgia (pain in the joints) or tenosynovitis (inflammation of a tendon
sheath) plus a pustular rash present on the extremities – especially the palms
and soles. N. gonorrhoeae can be isolated from the blood, but not usually
from the skin lesions.
b. Purulent Arthritis – most common in the knees, ankles, and wrists. N.
gonorrhoeae can be isolated from the pus aspirated from the joints.
c. Rarely endocarditis or meningitis
3. Associated with serum resistant strains (strains that are resistant to complement lysis)
and with poorly understood host factors. About 5% of people with disseminated
disease have deficiencies in the late acting components of complement (C5-C8).
5. Virulence Factors of N. gonorrhoeae (* indicates changeable virulence factors) (See Fig. 17-3)
1. Capsule – antiphagocytic
* * 2. Pili
a. Composed of repeating peptide subunits of pilin:
1. Predominantly of antigenically variable PilE
2. Small amounts of a tip associated PilC
b. Mediate initial attachment of gonococci to epithelial cells. (a two step
process - pili overcome the electrostatic barrier between the negatively
charged surfaces of the bacterial and host cell.)
c. Specific cell receptor unidentified; high specificity of binding for genito-
epithelial cells.
3. Outer Membrane Proteins
a. Por (protein I)
1. Most abundant outer membrane protein; extends through the
gonococcal cell membrane.
2. Forms trimers through which some nutrients enter the cell.
3. Role in virulence – when the gonococcal membrane is in intimate
contact with the host cell membrane, the Por protein is transferred to
the host cell, resulting in alterations in ionic permeability of the
host cell plasma membrane. May contribute to the intracellular
survival of gonococci inside of neutrophils?
** b. Opa (protein II) – opacity protein (describes the opaque morphology of
colonies growing on agar)
1. When expressed, is a major component of the cell surface.
2. Function in virulence - afimbrial adhesins that mediate firm
attachment of gonococci to epithelial cells. (2nd stage of attachment)
c. Rmp (protein III) – Reduction–modifiable protein – associates with Por in the
formation of pores in the cell surface. May protect other surface antigens (Por
protein, LOS) from bactericidal antibodies.
d. Tbp1, Tbp2 – two transferrin-binding proteins – that mediate acquisition of
iron directly from transferrin. (Not a siderophore system, but receptor
proteins for transferrin are induced in low-iron medium.)
e. Lpb – Lactoferrin-binding protein- mediates acquisition of iron
from lactoferrin.
In addition to d. and e., above, the gonoccoci are able to use free
hemoglobin and free heme to acquire iron. Additional iron regulated outer
membrane proteins are associated with these iron-acquiring mechanisms.
** f. LOS (Lipooligosaccharide) – endotoxin activity; thought to be responsible for
most of the symptoms of gonorrhea
B. Extracellular Products:
Secretory IgA 1 protease
C. Toxins – NONE!
6. Pathogenesis (See Figure 17-4)
A Entry via sexual contact
B. Superficial defenses of the mucosal surfaces are overcome.
C. Attachment and colonization of columnar epithelial cells at or near the site of inoculation.
1. Gonococci initially attaches to host epithelial cells via pili. Closer attachment is then
quickly mediated by the Opa outer membrane protein.
2. Secretory IgA protease – protects the organisms from antibodies present at the
mucosal surface?
3. In response to environmental conditions, iron repressed outer membrane proteins
produced which bind lactoferrin and transferrin, providing iron for growth.
D. Some epithelial cells are damaged by external colonization (due to gonococcal LOS in the
local environment), but others are probably invaded by the organism. (This is inferred by
experiments in vitro using organ cultures of human fallopian tubes.)
After attachment and initial colonization, the sequence of events probably includes:
1. Entry of gonococci into the host cell by endocytosis.
2. Intracellular replication inside the endocytic vesicle. (Host cell killing of bacteria within
the vesicles is inhibited by the membrane perturbing activities of Por?
3. Transport of the vesicle to the basal of the cell, fusion with the cell membrane, and
release of the gonococci into the subepithelial tissue- the lamina propria.
4. Multiplication in the lamina propria – aided by iron acquisition systems.
5. The gonococci have opportunities to spread because of the proximity of the lamina
propria to regional lymphatics and blood vessels; DGI happens in approx. 1% of
cases.
E. Host mounts a vigorous inflammatory response in the lamina propria (featuring the
phagocytes and complement) –which causes most of the disease symptoms.
1. LOS and other cell wall components elicit a strong inflammatory response that
probably gives rise to the local symptoms of purulent discharge and pain associated
with genital and rectal infections.
a. Both LOS and peptidoglycan cause the release of TNF alpha from a variety of
human cells (TNF alpha is associated with the death of ciliated cells in vitro in
the fallopian tube organ culture system).
b. Host cell lysis releases tissue factors that also contribute to inflammation.
c. Many gonococci are seen inside of phagocytes from urethral or cervical pus.
Are these bacteria killed by the phagocytes or do
some survive intracellularly?
1. The gonococcci are seen inside of phagolysosomes. It has been
demonstrated that degranulation has occurred, and that the
gonoccoci are dead inside the vacuoles.
2. However, also seen are some apparently damaged PMNs that
contain large numbers of gonococci being released.
3. In vitro experiments reveal that the vast majority of phagocytosed
organisms are killed and it is expected that most die in vivo.
However, there is no doubt that some will survive in the remains of
dead phagocytes. (GC – AN INTRACELLULAR PATHOGEN?)
These organisms are protected from host attack and can go on to
infect additional epithelial cells.
F. The “War with the Phagocytes” and Dealing with Complement
What other virulence factors do gonococci have that would allow them to (1) evade
phagocytosis, (2) stop the chemotaxis of phagocytes into the infected area, (3) kill the
phagocytes, or (4) resist complement opsonization and lysis (via the alternative
pathway)??? (See Below: strains that disseminate are serum resistant and effectively
resist complement lysis.)
FOLLOWING A LOCAL INFECTION, CERTAIN GONOCOCCI HAVE THE ABILITY TO
SPREAD AND CAUSE UPPER REPRODUCTIVE TRACT INFECTIONS OR DISSEMINATED
DISEASE
G. Survival in a New Host Environment- The Upper Reproductive Tract
1. Shifts in the surface components of the gonococci during this process
assist the gonoccci in the changing environments in vivo and help the
bacteria evade immune and phagocytic defenses. (For example, strains isolated from
the fallopian tube lack Opa proteins.)
2. Spread is also associated with host factors - Spread to the fallopian tubes occurs
mainly at the time of menstruation.
3. Pathogenesis of PID:
The ciliated cells are killed, and the resulting inflammatory response
(including release of TNF alpha) causes scarring and ultimately
fibrosis and stricture of the tubes, leading to PID.
H. Gonococcal strains that cause disseminated gonoccal infections (DGI)? – both
bacterial virulence factors and host factors both play a role in pathogenesis
1. Normal human serum is able to kill most circulating Gram negative organisms,
including N. gonorrhoeae. Why?
a. Serum contains the complement components and activation of complement
by the alternative pathway leads to opsonization by C3b and lysis by the
MAC.
b. Serum may contain IgM and IgG antibodies – which bind to the pathogen and
activate the classical complement pathway leading to opsonization by C3b or
lysis by the MAC. (The targets of the antibody are LOS and the major outer
membrane proteins.)
2. Strains of gonoccoci that disseminate are serum resistant – which means they are
able to resist the bactericidal effects of serum and are able to survive in the
bloodstream. How?
Due to Pathogen Factors:
a. The sugars that make up the hydrophilic part of the gonococcal LOS can
vary depending on the expression of enzymes that encode their synthesis.
Gonococci change both the length of these carbohydrate chains and the
sugars that make up the chains. These changes can make gonococci serum
resistant because the carbohydrate portion of LOS is involved in both
activation of complement and formation of the MAC.
b. Some N. gonorrhoeae strains attach a host-derived sialic acid residue the
terminal LOS galactose, a process called sialylation.
These bacteria become serum resistant because:
1. Sialic acid is a ubiquitous molecule in the host (and does
not activate complement)
2. The MAC does not form productively around the altered
LOS.
3. This modified LOS “cloaks” the pathogen in host
carbohydrates which blocks the action of bactericidal
antibodies directed against the LOS or the outer
membrane proteins (many of which are antigenic).
Due to Host Factors
a. Increased DGIs are associated with HIV infection.
b. Individuals deficient in the terminal complement components are predisposed
to recurrent systemic disease. WHY??
7. Antigenic Heterogeneity of N. gonorrhoeae (See Text pp. 163-165 and Lecture 10)
A successful vaccine for gonorrhea has not been developed because Neisseria gonorrhoeae is
able to vary the antigenic make-up of many of its cell surface molecules. It is believed this
variation allows the gonococci (1) to keep one step ahead of the host's humoral immune
response, and (2) to alter its cell surface as it adapts to the changing environments
encountered in vivo (from attachment to the columnar epithelial cells, to invasion, to encounters
with phagocytes, to survival in the upper reproductive tract, and for some strains dissemination
throughout the body.)
A. A Review Of The Ways That Pathogens Change Their Virulence Factors – types of
Changes and regulation we have met in MIC 361. (See attached supplement.)
B. Ways that N. gonorrhoeae Changes Its Virulence Factors:
1. Antigenic variation – variations that change the amino acid composition of
surface proteins. Allows new variants to arise during the course of an infection, so
protective immunity is never generated.
Previous examples of antigenic variation presented in this course include B.
recurrentis (relapsing fever), African Trypanosomes (African Sleeping Sickness)
(See Lecture 10) and Influenza viruses (see Lecture 20). These pathogens vary their
cell surface structures in direct response to antibodies produced by the host.
Antigenic variations seen with N. gonorrhoeae occur at high frequency in the
population and are not necessarily in response to the production of specific antibodies.
As a result, new variants continually emerge and the immune system is never able to
make an effective antibody response.
N. gonorrhoeae uses antigenic variation to vary its:
a. Pili (PilE) (See Text p. 165)
This pathogen changes both the amount of pilin subunit produced and the
amino acid composition of the subunits.
1. The amount of PilE produced is controlled at the level of transcription by a
two component regulatory system (PilA , a transcriptional sensor protein,
and PilB, an activator protein). The signal recognized by PilB is unknown.
2. The amino acid composition of PilE is varied in a process of
homologous recombination. Gonococci have multiple copies of the pilin
gene scattered around the chromosome. Only one of these copies has a
promoter and is expressed as a complete gene (pil E); the others are
nonexpressed copies (pil S). By homologous recombination with pil E genes
on the same chromosome (or from DNA taken up by transformation), all or
part of a silent, pil E gene is transferred to the pil E site and a new form of
pilin is expressed. Thus the gonococci can express an infinite variety of
pili by using a limited number of genes.
b. Opa proteins – the amino acid composition of Opa proteins is also varied by
a process of homologous recombination.
Most N. gonorrhoeae strains have multiple copies of the Opa gene, as many as 12,
which differ primarily in two hypervariable regions. Homologous recombination can
occur between the different copies of the gene to cause antigenic varians. (NOTE:
Phase variation (below) is a more common way for gonococci to vary these outer
membrane proteins.)
2. Phase variation – the expression of a protein is turned on and off at high
frequency.
a. Why would as pathogen want to turn on and off the expression of its
surface structures??
b. The E. coli example
Pathogenic strains of E. coli use site-specific inversion of a DNA segment
which bears a transcriptional promotor in order to turn on and off the
expression of their fimbriae- see Paradigm, Text p. 164)
c. Gonococci use multiple mechanisms to turn on and off the expression
of both pili and Opa proteins. Slipped strand mispairing is one of the
mechanisms used.
1. Pili (Pil+ ----- Pil -) (Phase variation occurs via multiple mechanisms,
including slipped strand mispairing - described below.)
2. Opa Proteins (Opa+ ---- Opa-)
3. Variations in LOS can render gonococci resistant to serum - See 6. H., above.
Changes in the length of the LOS carbohydrate chains or other changes in the
carbohydrate portion of LOS can render the gonococci serum resistant .
8. Laboratory Diagnosis
A. Direct Microscopy
1. Gram stain of purulent urethritis in males is > 90% sensitive and >95% specific in
detecting gonococcal infection in males.
2. This test is relatively insensitive in detecting infection in symptomatic and
asymptomatic females.
3. All negative results in men and women must be confirmed by culture.
B. Culture
1. Specimens
a. genital specimens (must collect specimen from endocervix correctly)
b. blood cultures (DGI)
c. joint fluid (DGI)
2. Media
a. Inoculated onto selective (Thayer-Martin) and nonselective (chocolate agar)
media.
b. Blood cultures only positive during the 1st week of infection with disseminated
disease.
3. Identification
a. Preliminary – oxidase +, Gram negative diplococci
b. Definitive – biochemicals
4. Antibiotic Sensitivity testing
C. Molecular Diagnosis
1. Commercial probes to detect the nucleic acids of N. gonorrhoeae or combination
assays for N. gonorrhoeae and Chlamydia are available. They are sensitive and
specific and give a rapid diagnosis. The drawback is the inability to monitor antibiotic
resistance.
9. Treatment
A. Penicillin is no longer the drug of choice. The concentration required to inhibit growth of N.
gonorrhoeae has steadily increased over the years. (In 1945: 200,000 units; currently 4.8
million units of Penicillin G are required.)
B. Penicillin resistance mediated by B lactamase initially reported from American servicemen
returning from southeast Asia, and is now worldwide. First reported case of PPNG in the U.S.
was 1976. Plasmid mediated.
C. Strains of penicillin-resistant N. gonorrhoeae that do not produce B lactamase have been
isolated. This chromosomally mediated resistance extends to tetracyclines, erythromycin,
and aminoglycosides.
D. Quinolone antibiotics such as ciprofloxacin has become the preferred drug in many areas of
the world where penicillin and tetracycline resistance are common. Now, gonococci resistant
to this class of drugs have been reported. While it is probably a matter of time before this
becomes widespread, for now, ciprofloxacin is still the drug of choice.
E. All patients with gonorrhea should be treated for chlamydial infection, because roughly 50% of
patients are coninfected with C. trachomatis..
10. Prevention
Infection rates can be reduced by avoiding multiple sexual partners, early diagnosis and treatment
of infected individuals (to rapidly eradicate the organism), finding cases and contacts through
education and screening populations at high risk.
A vaccine??? No current vaccine is available to confer some immunity against the wide range of
strains of gonococci and prevent asymptomatic infections.
Candidate surface molecules include:
1. Pili – not a viable vaccine candidate. Although pili have regions that are variable and
conserved, the conserved regions are buried in the interior of the pious and would not be
accessible to antibodies.
2. Por (Protein I) – 60% of outer membrane proteins- limited number of serotypes but weakly
immunogenic
3. LOS and Opa proteins– antigenic variation
4. The search is for an outer membrane protein produced by all N. gonorrhoeae strains.
SUPPLEMENT TO LECTURES 36 AND 37
WAYS THAT PATHOGENS CHANGE THEIR VIRULENCE FACTORS – types of changes and methods of regulation we have met in MIC 361.
1. CHANGES IN THE DNA SEQUENCE OF A GENE
a. Point Mutation – point mutations in the genes for HA and NA of Influenza virus result
in small variations in the HA and NA proteins - causes antigenic drift (Lecture 20)
b. Homologous Recombination – antigenic variation of pili in N. gonorrhoeae
Homologous recombination between a silent version of the gene from another part of
the chromosome (or on DNA taken up by transformation) and the version of the gene
currently being expressed can change the amino acid sequence of the pilin protein
being produced (Lectures 36 and 37).
c. Gene Reassortment – coinfection of a host cell with two different influenza viruses
can result in reassortment of the RNA segments and lead to progeny with different HA
and NA proteins than either parent - causes antigenic shift (Lecture 20).
d. Gene Switching –
1. African trypanosomes and Borrelia recurrentis – have many genes for their
major surface proteins but only express a single gene at time; for a gene to be
activated, it is first duplicated and then transposed into a transcriptionally
active expression site, displacing the previous gene. The result is a pathogen
with a completely new cell surface protein. (Lecture 10)
e. Gene Rearrangements can cause “Phase Variation” – switching between “on” and
“off” forms of a gene (at high frequency) (Lectures 36 and 37)
1. E. coli can turn “on” and “off” the production of pili by site specific inversion of
a DNA segment containing a promoter.
2. N. gonorrhoeae can turn “on” or “off” the production of Opa proteins by a
different mechanism.
The gene encoding the Opa protein has a series of short repeated DNA
sequences at its amino terminal end. The number of repeats determines
whether the rest of the gene is “in frame” (producing an active protein) or “out
of frame” (producing a truncated inactive protein.) During replication,
slipped-strand synthesis occurs and errors are made in the number of
copies of the repeats. The result is that some progeny will have genes
encoding the Opa protein while other progeny will not.
2. CHANGES IN THE NUMBER OF TRANSCRIPTS PRODUCED
(TRANSCRIPTIONAL REGULATION)
a. Transcriptional Regulation by a Repressor – Corynebacterium diphtheriae
The gene for diphtheria toxin is regulated by an iron-dependent repressor
protein, DtxR. When iron levels are high, the iron-bound form of DtxR binds
to the DNA at sites that overlap the promoter, blocking the binding of RNA
polymerase to the promoter and inhibiting transcription. (Lecture 14)
b. Transcriptional Regulation by an Activator(s) (singly or in a two
component system)
a. Bordetella pertussis- coordinately regulates several of its virulence
factors with a two component regulatory system (two activators are
used to turn on virulence genes sequentially).
A transmembranous sensor protein with a kinase activity responds to
an environmental signal and is autophosphorylated. The
phosphorylated sensor protein then activates the cytoplasmic
response regulator protein by phosphorylation. The phosphorylated
form of the response regulator protein activates transcription of the B.
pertussis virulence genes. (Lecture 15)
b. Vibrio cholerae – coordinately regulates its virulence factors with a
two component regulatory system. In this system, a single
transmembranous protein (ToxR) acts as both the sensor and the
regulator protein leading to increased transcription of V. cholerae
virulence genes. (Lecture 32)
3. CHANGES IN THE AMOUNT OF GENE PRODUCT (POSTTRANSCRIPTONAL
REGULATION)- Some virulence genes are regulated at the level of translation.
a. Posttranslational activation – the form of the protein translated from the
mRNA may require some sort of posttranslational processing to become
active. After translation, several A-B exotoxins must be proteolytically nicked
or cleaved to become activated.