In this section, you will explore the following questions:

  • How do single-celled yeasts use cell signaling to communicate with each other?
  • How does quorum sensing allow some bacteria to form biofilms?

Connection for AP® Courses

Cell signaling allows bacteria to respond to environmental cues, such as nutrient levels and quorum sensing (cell density). Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. For example, budding yeasts often release mating factors that enable them to participate in a process that is similar to sexual reproduction.

Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.D Cells communicate by generating, transmitting and receiving chemical signals.
Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.
Science Practice 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain.
Learning Objective 3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response.
Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.
Science Practice 6.1 The student can justify claims with evidence.
Learning Objective 3.37 The student is able to justify claims based on scientific evidence that changes in signal transduction pathways can alter cellular response.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 3.31][APLO 3.37]

Within-cell signaling allows bacteria to respond to environmental cues, such as nutrient levels. Some single-celled organisms also release molecules to signal to each other.

Signaling in Yeast

Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. Budding yeasts (Figure 9.16) are able to participate in a process that is similar to sexual reproduction that entails two haploid cells (cells with one-half the normal number of chromosomes) combining to form a diploid cell (a cell with two sets of each chromosome, which is what normal body cells contain). In order to find another haploid yeast cell that is prepared to mate, budding yeasts secrete a signaling molecule called mating factor. When mating factor binds to cell-surface receptors in other yeast cells that are nearby, they stop their normal growth cycles and initiate a cell signaling cascade that includes protein kinases and GTP-binding proteins that are similar to G-proteins.

The photo shows yeast cells, some of which have buds protruding from them.
Figure 9.16  Budding Saccharomyces cerevisiae yeast cells can communicate by releasing a signaling molecule called mating factor. In this micrograph, they are visualized using differential interference contrast microscopy, a light microscopy technique that enhances the contrast of the sample.

Signaling in Bacteria

Signaling in bacteria enables bacteria to monitor extracellular conditions, ensure that there are sufficient amounts of nutrients, and ensure that hazardous situations are avoided. There are circumstances, however, when bacteria communicate with each other.

The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reaches a certain level, specific gene expression is initiated, and the bacteria produce bioluminescent proteins that emit light. Because the number of cells present in the environment (cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing. In politics and business, a quorum is the minimum number of members required to be present to vote on an issue.

Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules such as acyl-homoserine lactone (AHL) or larger peptide-based molecules; each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off (Figure 9.17). The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.


VISUAL CONNECTION

The left part of this illustration shows a single bacterial cell. The cell produces autoinducers, which diffuse away from the cell and cannot bind the intracellular receptor. The right part of this illustration shows many bacterial cells. More autoinducers are present, which bind receptors that in turn bind DNA and regulate the expression of certain genes. Autoinducer gene expression is turned on, resulting in a positive-feedback loop.
Figure 9.17  Autoinducers are small molecules or proteins produced by bacteria that regulate gene expression.
 
Which of the following statements about quorum sensing is false?
  1. Autoinducers must bind to receptors to turn on transcription of genes responsible for the production of more autoinducers.
  2. Autoinducers can only act on a different cell. It cannot act on the cell in which it is made.
  3. Autoinducers turn on genes that enable the bacteria to form a biofilm.
  4. The receptor stays in the bacterial cell, but the autoinducers diffuse out.

 Some species of bacteria that use quorum sensing form biofilms, complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host. Bacterial biofilms (Figure 9.18) can sometimes be found on medical equipment; when biofilms invade implants such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.

The ability of certain bacteria to form biofilms has evolved because of a selection of genes that enable cell-cell communication confers an evolutionary advantage. When bacterial colonies form biofilms, they create barriers that prevent toxins and antibacterial drugs from affecting the population living in the biofilm. As a result, these populations are more likely to survive, even in the presence of antibacterial agents. This often means that bacteria living in biofilms have higher fitness than bacteria living on their own.

SCIENCE PRACTICE CONNECTION FOR AP® COURSES

THINK ABOUT IT

Why is signaling in multicellular organisms more complicated than signaling in single-celled organisms such as microbes?

EVERYDAY CONNECTION

Part a: This electron micrograph shows a film of bacteria. Part b: This photo shows a Hawaiian bobtail squid.
Figure 9.18  Cell-cell communication enables these (a) Staphylococcus aureus bacteria to work together to form a biofilm inside a hospital patient’s catheter, seen here via scanning electron microscopy. S. aureus is the main cause of hospital-acquired infections. (b) Hawaiian bobtail squid have a symbiotic relationship with the bioluminescent bacteria Vibrio fischeri. The luminescence makes it difficult to see the squid from below because it effectively eliminates its shadow. In return for camouflage, the squid provides food for the bacteria. Free-living V. fischerido not produce luciferase, the enzyme responsible for luminescence, but V. fischeri living in a symbiotic relationship with the squid do. Quorum sensing determines whether the bacteria should produce the luciferase enzyme. (credit a: modifications of work by CDC/Janice Carr; credit b: modifications of work by Cliff1066/Flickr)
 
Free-living V. fischeri do not luminesce. Why?
  1. The squid provides certain nutrients that allow the bacteria to luminesce.
  2. The squid produces the luminescent luciferase enzyme, so bacteria living outside the squid do not luminesce.
  3. The ability to luminesce does not benefit free-living bacteria, so free-living bacteria do not produce luciferase.
  4. Luciferase is toxic to free-living bacteria, so free-living bacteria do not produce this enzyme.

Research on the details of quorum sensing has led to advances in growing bacteria for industrial purposes. Recent discoveries suggest that it may be possible to exploit bacterial signaling pathways to control bacterial growth; this process could replace or supplement antibiotics that are no longer effective in certain situations.

LINK TO LEARNING

Watch geneticist Bonnie Bassler discuss her discovery of quorum sensing in biofilm bacteria in squid.

What does bioluminescence show about communication in bacteria?
  1. Bacteria interact by physical signals among a colony.
  2. Bacterium interact by chemical signals when it is alone.
  3. Bacterium interact by physical signals when it is alone.
  4. Bacteria interact by chemical signals among a colony.

EVOLUTION CONNECTION

The first life on our planet consisted of single-celled prokaryotic organisms that had limited interaction with each other. While some external signaling occurs between different species of single-celled organisms, the majority of signaling within bacteria and yeasts concerns only other members of the same species. The evolution of cellular communication is an absolute necessity for the development of multicellular organisms, and this innovation is thought to have required approximately 2.5 billion years to appear in early life forms.

Yeasts are single-celled eukaryotes, and therefore have a nucleus and organelles characteristic of more complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans illustrate the evolution of increasingly complex signaling systems that allow for the efficient inner workings that keep humans and other complex life forms functioning correctly.

Kinases are a major component of cellular communication, and studies of these enzymes illustrate the evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development, differentiation, and cellular communication used in multicellular organisms.

Because yeasts contain many of the same classes of signaling proteins as humans, these organisms are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to study, although they contain similar counterparts to human signaling. 2

Based on the Evolution Connection, which of the following best describes the evolution of kinases?
  1. The tyrosine kinases evolved before yeast diverged from other eukaryotes, but the other fifty-five subfamilies of kinases evolved after yeast diverged.
  2. Fifty-five subfamilies of kinases evolved before yeast diverged from other eukaryotes, but the tyrosine kinases evolved after yeast diverged.
  3. All kinases evolved in yeast, but yeast later lost the tyrosine kinases because they do not need them.
  4. The evolution of tyrosine kinases involved in cellular communication occurred about 2.5 billion years ago.

LINK TO LEARNING

Watch this collection of interview clips with biofilm researchers in “What Are Bacterial Biofilms?”

Recurrent urinary tract infections occur when the urinary tract becomes reinfected by the same bacteria. Why are recurrent urinary infections difficult to treat?
  1. Bacteria often form biofilms in recurrent infections and these may be more antibiotic resistant.
  2. Bacteria rarely form biofilms in recurrent infections, making them more resistant to antibiotics than if they were not in a biofilm.
  3. Bacteria produce biofilms which behave like a unicellular organism.
  4. Bacteria don't produce biofilms in recurrent infections but become resistant due to repeated exposure to antibiotics.

Footnotes

  • G. Manning, G.D. Plowman, T. Hunter, S. Sudarsanam, “Evolution of Protein Kinase Signaling from Yeast to Man,” Trends in Biochemical Sciences 27, no. 10 (2002): 514–520.