What magnification is used to view bacteria under a microscope?

Microorganisms – tiny creatures too small to see with the naked eye – are everywhere. But if you want to get a glimpse into this microscopic kingdom, you’re going to need a powerful microscope.

There are a huge variety of microorganisms in any environment – from fungi to protozoans and bacteria.

Naturally, microorganisms also come in a range of different sizes. Before you grab any old microscope, you should make sure that it has enough magnification to let you see the organism you’re looking for.  

If you’re interested in getting a good look at bacteria, you’re going to need a compound microscope with a high level of magnification.

Most bacteria are small and hard to see, with an average size somewhere between 0.2 and 2.0 μm (micrometers). They also tend to be transparent and sometimes clump together – this can make it quite difficult to properly see your specimens.

However, with high magnification, a bit of patience, and possibly a little extra help from stains, you’ll be able to see your bacteria in glorious detail.

The right magnification

To get your first glimpse of bacteria, you need a magnification of 100x. At this level, bacteria start to become visible as very small dots in your image. You won’t be able to see any detail though, so this isn’t very helpful.

At 400x magnification, you can start to see the shape of your bacteria. You’ll also be able to see them moving around. However, you still won’t be able to see enough detail for a good inspection.

At 1000x magnification, you finally get a reasonable look at your tiny friends. You’ll be able to distinguish between different bacteria and get a look at their internal structure.

Despite having the appropriate magnification, you might still struggle to properly see your bacteria. This is because bacteria are largely transparent. Even if they do have natural pigment, there isn’t enough of it to provide good visibility and contrast.

Fortunately, there are a few solutions to this problem.

How to make your bacteria more visible

There are two general approaches to the problem of transparent microorganisms.

Staining

The first approach is to stain the bacteria – you bathe your bacteria in a special dye (or dyes) so that they’re easier to see under a microscope.

There is a huge range of staining options, and they can even be used to differentiate between species of bacteria by dyeing them with different colours.

While you would usually stain the bacteria directly, you can sometimes use what’s known as a ‘negative’ stain, which colours the background instead of the bacteria. This lets you see the bacteria as a silhouette against a colourful backdrop.

The main downside to staining is that it often kills the bacteria. So, if you’re particularly interested in the movement or metabolism of your bacteria, a different approach is called for. This is phase contrast.

Phase contrast

A (but more expensive) option.

When light passes through bacteria, it is slightly refracted. Phase contrast microscopes use a special lens and condenser to convert that refracted light into much greater contrast.

You’ll be able to see bacteria and their internal structure and components much more easily using phase contrast. It has the added benefit of not harming your specimens, so you’ll be able to watch them moving about freely and living their lives.

The only real downside to phase contrast is the cost. Phase contrast microscopes are more expensive, and the kits to upgrade your existing microscope to use phase contrast can range from around $500 for a ACHROMATIC Phase Contrast Kit to over $1500, so this approach might be too pricey for a casual hobbyist.

Still, phase contrast is a great option if you can afford it.

Beware magnification over 1000x

You might think that if 1000x magnification gives you a good look at your bacteria, then a higher magnification will be even better.

Not so.

Microscope objective lenses have a numerical aperture (NA) value – this represents the resolution of the lens, or the amount of detail you can see.

If you take that NA value and multiply it by 1000, you’ll get the maximum useful magnification you can use that lens for. If the total magnification you’re currently using is higher than that maximum, you’re going to start running into a problem known as empty (or false) magnification.

Trying to use a higher magnification without enough resolution will produce a blurry and distorted image. You won’t be able to see anything properly, let alone useful detail.

In general, most microscopes won’t be able to go over 1000x magnification without losing visual quality. Be wary of microscopes claiming to go beyond that limit, particularly for a relatively cheap price.

Tips

  • Try not to use too much water/liquid on your slides – it can be hard to find and focus on your bacteria if they’re floating around.
  • If you have a condenser, close the diaphragm – this will give you higher contrast and can help you see your bacteria.
  • If you’re seeing only big ‘blobs’ of bacteria, the density might be too high – try diluting your sample.

 

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