Microbe Spotlight: Week 1

NDSU Microbiology
by NDSU Microbiology

The students in Infectious Disease Pathogenesis Lab will be blogging about the strange and wonderful this semester. This week's guest bloggers are: Tyler Anderson, Ellie Bajczyk, and Ellyn Andrews. Check back next week for a new selection of microbe spotlights.

Agrobacterium tumefaciens

Agrobacterium tumefaciens is a Gram-negative rod containing one to six circumthecal flagella. It grows in aerobic conditions and can freely live in soil. A. tumefaciens carries both a circular chromosome and linear chromosome as well as two plasmids. Although this bacterium is not harmful to humans, it is pathogenic to plants, causing the slow and distorted growth of crown gall disease in many different plants, including: walnuts, grapes, and sugar beets.

In order for it to be virulent, A. tumefaciens must contain a tumor-inducing plasmid (Ti plasmid). The Ti plasmid contains T-DNA (transfer DNA) along with all the other necessary genes for transferring the plasmid to the plant cell. When the bacterium recognizes phenolic compounds of the plant cell, the virulence genes in the Ti plasmid are expressed. These genes turn off the flagella when in contact with the plant cell, allowing the bacterium's T pilus to attach to the plant cell. A. tumefaciens then infects the plant’s chromosomal DNA with its Ti plasmid. The bacteria can cycle from the biofilm on rhizoplane of plants to the soil where it can affect nearby plants. It performs this cycle by forming loose galls on infected plants that may fall off into the soil where they will find a new host plant to infect.

Although harmful to plants in nature, there are beneficial uses for the bacterium in the laboratory. A. tumefaciens is greatly studied to investigate plant genetics and crop modification. To do this, foreign genes can be engineered into the bacterial Ti plasmid DNA and inserted into a plant's genome. As long as the bacterium can be controlled in nature from harming plants, it may remain a useful study source for a variety of plants in the laboratory.

By Tyler Anderson

Image Source: A.G. Matthysse, K.V. Holmes, R.H.G. Gurlitz


The genus, Burkholderia, encompasses over 30 different bacterial species and is responsible for diseases, such as glanders, found in livestock, humans and other mammals. Glanders is caused by B. mallei and mainly infects the lungs and airways of horses, but can also affect humans. Since there is no vaccine against B. mallei, and it can be passed by close contact and inhalation, the CDC classifies it as a Category B critical biological agent that could be harnessed for use in biological warfare. In fact, in World War I, Germany did just that, infecting and killing the livestock of the Allies.

B. cepacia is infamous for infecting those with cystic fibrosis leading to severely decreased lung function and death. Originally an inhabitant of soil and humid environments, B. cepacia was first discovered as the causative agent of onion rot. Coincidentally, the unique properties of this microbe make it a potential agent for biodegradation and for influencing and promoting crop growth. However, because of its high mutation rate and unpredictability, it is not possible to control the strains in nature to prevent them from harming humans.

So although Burkholderia could potentially be an economically and environmentally useful pathogen if controlled and used in an appropriate way, this is not likely to happen in the near future. Burkholderia species are dangerous and their effects fatal, so until scientists can accurately predict and manipulate the outcome, Burkholderia is a genus to keep a close eye on.

By Ellie Bajczyk

Image Source: Gavin Koh

Bacillus thuringiensis

Bacillus thuringiensis is a Gram-positive, spore-forming bacterium that is naturally occurring. It thrives in soil and infects and kills insects. It has been genetically modified to infect a wide variety of insects and is arguably one of the most popular and widely utilized pesticides in the world.

This bacterium kills so effectively because it forms different crystal proteins specific for different insects. These proteins work by essentially exploding the epithelial cells of the insect gut and digestive tract. These symptoms previously observed in insects have recently sprung up in humans as a “mystery illness,” sometimes called leaky gut syndrome, with seemingly no traceable cause. The possibility that this syndrome is linked to the use of B. thuringiensis in agriculture has spurred concern among some researchers and the general public.

The use of B. thuringiensis has also caused some insects to become resistant to the pesticide. It has also been responsible for killing numerous colonies of natural insect predators, which do the job of pesticides in fields. Overall, B. thuringiensis is a fantastic tool for keeping crops healthy; however, it is my opinion that the use/overuse of genetically-modified strains poses an interesting ethical question: “Was all this worth it to kill a fly?”

By Ellyn Andrews

Image Source: P.R. Johnston