Not All Mice Are Created Equal When it Comes to Gonorrhea

by E. Ohneck

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection, gonorrhea, is a Gram-negative diplococcus and an obligate human pathogen. An estimated 800,000 cases of gonorrhea occur each year in the United States (1). The most common sites of infection are the cervix and the male urethra, and symptomatic infection is characterized by a purulent exudate composed of numerous polymorphonuclear leukocytes (PMNs) containing intracellular gonococci (3).

The development of whole model systems to study N. gonorrhoeae infection is important, as several different cell types are involved in host response. Currently, infection in males is examined through urethral inoculation of male volunteers, and results indicate a strong innate immune response characterized by induction of proinflammatory cytokines (3). Due to the risk of serious complications and the frequency of asymptomatic infection in women, such volunteer infection models cannot be utilized for female infection, and studies are currently limited to tissue culture cells and a developing 17β estradiol-treated BALB/c mouse model (3).

To better characterize the innate immune response to gonococcal infection in BALB/c mice, and to determine if response to infection varies among different inbred mouse strains, Packiam et al analyzed the ability of BALB/c, C57BL/6, and C3H/HeN mice to support vaginal colonization by N. gonorrhoeae and mount an inflammatory response. Their results show the establishment of a productive infection in BALB/c mice characterized by a large influx of PMNs and the induction of proinflammatory cytokines, including TNFα, IL-6, and MIP-2, a homolog of human IL-83.

C57BL/6 mice, however, while able to support colonization by N. gonorrhoeae to the same levels as BALB/c mice, did not mount an inflammatory response, and thus might provide a model for asymptomatic infection (3). Finally, C3H/HeN mice were inherently resistant to N. gonorrhoeae by a mechanism other than an overwhelming innate response, as shown by their inability to support colonization and a lack of PMN influx upon infection (3). Taken together, these results suggest an import role for host genetic background in determining susceptibility to N. gonorrhoeae.

This phenomenon is not restricted to N. gonorrhoeae. In fact, variation in vulnerability to specific pathogens among different inbred mouse strains has been observed for numerous bacteria, including Mycobacterium tuberculosis, Salmonella typhimurium, and Legionella pneumophila (2).

These results, therefore, raise several important questions. What genetic factors mediate susceptibility to specific pathogens in otherwise healthy individuals? What bacterial systems and components are involved in determining host genetic background-specific susceptibility? How can we exploit these host-pathogen relationships in the development of more efficient therapies and vaccines?

Bacterial disease is often studied from two distinct angles: microbial mechanisms of pathogenesis and virulence, and host defenses against microbial invaders. Currently, however, more emphasis is being placed on examining the host-pathogen interface, and studies such as this elucidate the demand for the breakdown of the barrier between microbiology and immunology. We cannot fully understand microbial disease mechanisms without taking into consideration host, pathogen, and the complex interactions that mediate their relationship.

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Sources:
1. CDC. Increase in fluoroquinolone-resistant Neisseria gonorrhoeae among men who have sex with men—United States, 2003, and revised recommendations for gonorrhea treatment, 2004. MMWR Morb Mortal Wkly Rep 2004; 53: 335 – 338.

2. Kramnik I and Boyartchuk V. Immunity to intracellular pathogens as a complex genetic train. Curr. Opion. Microbiol. 2002; 5:111 – 117.

3. Packiam M, Veit SJ, Anderson DJ, Ingalls RR, & Jerse AE (2009). Mouse Strain-dependent Differences in Susceptibility to Neisseria gonorrhoeae Infection and Induction of Innate Immune Responses. Infection and immunity PMID: 19901062

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Other Articles of Interest:
Out With the Bad: Efflux in Klebsiella
Nitric Oxide Synthase Isn't Just Used By Our White Blood Cells
A Home for the Bugs in Our Appendix

Noisy and Bistable Gene Expression: Why Genes and Environment Aren't Everything

by Tim

Classically, a cell's phenotype was thought to be a product of its genetic background and its environment. All changes within a cell would be due to the cell's genetic capability to react to the environmental changes happening around them. However, as we begin looking more in depth at cell populations at the single cell level, we are finding that this paradigm isn't always true.

In this two part series, I want to examine how genetically identical cells in equal environments can undergo very different developmental changes. Specifically, we will look at the induction of competence in Bacillus subtilis and the development of persister cells in E. coli. In both of these cases, the cells are genetically and environmentally identical. But in each case, a subset of the population undergoes a drastically different developmental change.

As Bacillus begins to enter stationary phase, a subpopulation of cells begins to become competent, allowing a small percentage of cells to take up DNA from the environment. This process is activated by the ComK protein, which also activates its own expression in a positive feedback loop. As ComK levels increase, a threshold is hit, leading to a rapid increase in ComK production, and a subpopulation of cells begin to enter competence. This leads to the question as to the mechanism which allows only a subset of cells to enter competence.

Back in 2007, the Dubnau Lab at UMDNJ published a paper in Science discussing how very slight variations, or noise, in the amount of comK mRNA could lead to certain cells becoming competent, while others remained vegetative. The authors used fluorescent in situ hybridization to measure the exact amount of comK mRNA in individual cells. They found that in early stationary phase the number of comK mRNAs in the total population increased from 0.7 to 1 per cell. As the population average increased ever so slightly, a subpopulation began switching to competence. Thus, as the average was shifted, a small subset was well-enough above the mean to hit the ComK threshold and enter competence. Noise in gene expression dictated which cells became competent.

Those cell to cell variations of comK expression could be due to intrinsic noise, events such as mRNA decay or transcription initiation rates, or extrinsic noise such as the concentration of polymerases or transcription factors. To test which of these contributed to comK variability, the authors again used the in situ hybridization technique to measure comK mRNA as well as a control mRNA under a comK promoter. Extrinsic variations should affect both mRNAs equally, while intrinsic variations would only be acting on a single locus. Counting the number of mRNAs immediately before the induction of competence, they found that the number of each mRNA type was uncorrelated, indicating that random intrinsic noise was responsible for the variations in comK expression.

So what does this all mean?

In essence, we must first understand that when we see a population with our favorite gene (OFG) expressed at level "X," really "X" just represents the mean expression of the population, and that per cell, the expression of this gene falls somewhere in a normal curve around "X." Cells with slightly higher or lower levels of OFG can end up having large effects on phenotypic outcomes, particularly if OFG is a regulator.

Importantly, it shows that not all phenotypic outcomes are entirely the sum of genetics and environment, but can be due to completely random events within the cell.

It also raises the question as to why the whole population would only want a subset to become competent. Perhaps it is a form of the bacteria hedging their bets, allowing only a small percentage to take a great risk (i.e. bringing in harmful DNA, delaying sporulation) to receive great reward (new genetic information to outcompete others), while the remaining population continues laissez faire. But this can be a topic for next week when we take a look at bet-hedging in persister cells.

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Source:
Maamar H, Raj A, & Dubnau D (2007). Noise in gene expression determines cell fate in Bacillus subtilis. Science (New York, N.Y.), 317 (5837), 526-9 PMID: 17569828

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Other Articles of Interest:

Altruism in Bacteria? Allowing Yourself to Die for the Good of The Species

Prokaryotes Can Do Geometry, and Even Have Their Own Protractor

Throwing the Clutch (not the Brake) on a Bacterial Flagella

Antiobiotic Treatment: Increasing the Rates of Genetic Exchange